surveillance of surgical site infections in nhs hospitals in england

Surveillance of Surgical Site Infections in NHS Hospitals in England2011/2012

Surveillance of Surgical Site Infections in NHS hospitals in England 2011/2012

Suggested Citation

Health Protection Agency. Surveillance of surgical site infections in NHS hospitals in England, 2011/2012. London: Health Protection Agency, December 2012. Available from: www.hpa.org.uk

This report from the Health Protection Agency reflects understanding and evaluation of the current scientific evidence as presented in this document.

This report was authored by S Elgohari with the assistance of S Thelwall, T Lamagni, L Sheridan, A Charlett and A Saie. We extend our thanks to the administrative staff at the Surgical Site Infection Surveillance Service (SSISS) and the considerable contribution made by NHS Trusts in England who have devoted time and effort in collecting these data. Front cover image (left): with kind permission from North West London Hospitals NHS Trust, London, UK.

Contents

Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Key points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1 Introduction and surveillance methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 Data collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4 Changes to surgical categories under surveillance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.5 Analyses presented in this report. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1 Hospital participation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Data quality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Patient and operation-related characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.4 Rate of surgical site infection by surgical category . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 Risk factors for SSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Trends in rate of SSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.1 National trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.2 Hospital-level trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5 Characteristics of surgical site infections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.1 Type of SSI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.2 Causative pathogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

6 Variation in rates of SSI between NHS hospitals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6.1 Box and whisker plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6.2 Funnel plots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

8 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

9 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29


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2


Key points

• Data on 496,373 procedures and 6,995 inpatient and readmission surgical site infections (SSIs) from 17 surgical categories were collected by 235 NHS hospitals and independent sector treatment centres between April 2007 and March 2012.

• Since the introduction of mandatory orthopaedic SSI surveillance in 2004, there has been a 3-fold increase in the number of procedures submitted and a 2-fold increase in the number of participating hospitals.

• For the mandatory surveillance of orthopaedic surgery, 148 NHS Trusts and 10 NHS Treatment centres participated in 2011/12 contributing data on 95,938 procedures. Two Trusts did not participate in the mandatory surveillance in this year.

• The proportion of hospitals undertaking continuous surveillance in 2011/12 was highest in hip prosthesis and knee prosthesis (>50%) and lowest in vascular surgery and abdominal hysterectomy (<25%).

• Data completion was close to 100% overall for most of the key risk factors (age, patient sex, duration of operation and wound class). The ASA score (pre-operative physical status) was slightly lower at 93%. BMI had the lowest data completion rate at 42% overall.

• The risk of SSI varied according to the likelihood of microbial contamination at the operative site, being highest in large bowel surgery at 10% and lowest in knee prosthesis surgery at <1%.

• The risk of SSI increased consistently across all surgical categories where the patient’s ASA score was ≥3, the duration of operation was above 75th percentile for that category, the wound class was contaminated/dirty or where the surgery was undertaken as an emergency procedure.

• The national trends in the inpatient/readmission SSI rate between 2008/09 and 2011/12 indicated that a significant decrease occurred in hip prosthesis, bile duct/liver/pancreatic surgery and coronary artery bypass graft (the latter from 2009/10).The inpatient/readmission SSI trends at hospital level were examined for seven categories. The majority of hospitals exhibited a stable or decreasing trend in rates of infection over time.

• A significantly increasing trend in SSI was found at a national level for patients undergoing knee prosthesis, large bowel and cholecystectomy surgery. Investigating the drivers behind these increases should be considered a priority.

• Three NHS Trusts were identified as high outliers in 2011/12 for the mandatory orthopaedic surveillance with an incidence of SSI higher than expected nationally. Nine NHS Trusts were identified as low outliers with an incidence of SSI lower than expected. Trusts have been contacted and asked to undertake further investigations.

• The majority of causative pathogens in 2011/12 were Enterobacteriaceae accounting for 29% of inpatient and readmission SSI isolates. S. aureus accounted for 24% of isolates. Meticillin-resistant S. aureus (MRSA) accounted for 4% of total isolates and 18% of S. aureus isolates representing a continuing decrease from previous years.

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1 Introduction and surveillance methods

1 .1 Introduction

This report is a summary of data on surgical site infections (SSIs) collected by NHS hospitals and independent sector NHS Treatment Centres in England participating in one of 17 surgical categories of surveillance between April 2004 and March 2012. Results presented in this report include orthopaedic data submitted by hospitals as part of the mandatory requirement introduced by the Department of Health in April 2004[1]. This report also includes data on other surgical categories submitted on a voluntary basis. For the mandatory orthopaedic surveillance, all NHS Trusts undertaking orthopaedic surgical procedures must carry out a minimum of three months’ surveillance in a financial year in at least one of four categories (hip prosthesis, knee prosthesis, repair of neck of femur or reduction of long bone fracture). NHS Trusts are required to collect data from at least one hospital site within their Trust. Trusts with very small volumes of orthopaedic surgery are exempt from the mandatory surveillance but are expected to undertake surveillance in a category of their choice that best reflects surgical activity.

1 .2 Data collection

All hospitals participating in HPA SSI surveillance are required to follow the surveillance protocol outlining the definitions for identifying cases and follow-up methods. Training is provided to hospital staff to facilitate compliance with the surveillance protocol. Each hospital collects data prospectively on all eligible patients in a self-selected surgical category over a 3 month period. A set of demographic and operation-related data are collected for each eligible procedure, along with details of any infections meeting the case definition, and submitted via a secure web-based data entry screen. Patients were followed up for 30 days following the operation where a procedure did not involve an implant and one year for procedures with an implant. Procedures performed solely by endoscopic or by laparoscopic approaches are excluded from the surveillance.

Since July 2008, hospitals have been required to have systems in place to identify patients who are included in the surveillance and readmitted to hospital with an SSI. Patients readmitted to another hospital with an SSI are attributed to the hospital in which the patient originally had the operation. Two other post-discharge surveillance methods were introduced in 2008 but remain optional (and are excluded from the analyses in this report): a) systematic review of patients either returning to hospital in an outpatient clinic or reviewed at home by clinical staff trained to apply the surveillance definitions and b) wound healing questionnaires completed by patients 30 days following the operation[2].

1 .3 Definitions

For the purposes of surveillance, SSIs are based on a standard set of clinical criteria. Infections are categorized into those that affect superficial tissues (skin and subcutaneous layer) of the incision and those that affect the deeper tissues (deep or organ-space) according to the Centers for Disease Control and Prevention (CDC) definitions[3]. One of the key risk factors collected by hospitals is the ASA score. This is a classification system developed by the American Society of Anesthesiologists to classify a patient’s pre-operative physical status on a scale from one to five with higher scores indicating severe systemic disease.

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1 .4 Changes to surgical categories under surveillance

Changes were made to the following categories during the period covered by this report:

•HiphemiarthroplastywasdiscontinuedinJuly2008andreplacedwithrepairofneckoffemur.

•OpenreductionoflongbonefracturewasdiscontinuedinJuly2008andreplacedwithreduction of long bone fracture.

•SpinalsurgerywasintroducedinJuly2008.

•Breast,cranialandcardiac(non-coronaryarterybypassgraft,CABG)surgerywereintroduced in April 2010.

1 .5 Analyses presented in this report

Data for this report were extracted on 27th September 2012. Data collected between April 2004 and March 2012 by participating NHS hospitals and independent sector NHS Treatment Centres were included. To evaluate trends, data across this entire period were analysed. For benchmarking purposes, five-year cumulative data were used (April 2007 to March 2012), whereas data for April 2011 to March 2012 were used to describe recent activity.

The cumulative incidence of SSI, or the rate of SSI (%), presented in this report is based on SSIs detected during hospital stay combined with SSIs identified on readmission following the initial operation. Where appropriate, inpatient SSIs are analysed separately for meaningful interpretation. To take into account the variation in the length of follow-up during the hospital stay, the incidence density was calculated using the total number of days of patient follow-up in the denominator giving the number of SSIs per 1,000 patient days of follow-up.

Funnel plots were constructed for each mandatory orthopaedic category using the inpatient/readmission SSI rate per NHS Trust during 2011/12. These plots identify Trusts whose rates fall within the expected variation and Trusts whose rates are significantly higher or lower than expected nationally (‘outliers’). Independent sector NHS treatment centres are included in these plots. An additional supplement to this report presents the SSI rates in 2011/12 by NHS Trust and orthopaedic category:

http://www.hpa.org.uk/webc/HPAwebFile/HPAweb_C/1317137170118

www.hpa.org.uk/Topics/InfectiousDiseases/InfectionsAZ/SurgicalSiteInfection/

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2 Overview

2 .1 Hospital participation

Overall, the number of hospitals participating in the national SSI surveillance scheme has increased since the implementation of the mandatory surveillance of orthopaedic categories in April 2004 (Figure 1). The volume of operations has increased three-fold between April 2004 and March 2012, with approximately 30,000 records per quarter submitted in 2011/12. Overall, 197 hospitals from 152 NHS Trusts participated in SSI surveillance together with 10 independent NHS treatment centres in 2011/12. For the mandatory orthopaedic surveillance, 186 NHS hospitals from 148 NHS Trusts participated together with 10 independent sector NHS treatment centres.

After excluding eight categories with a small number of participating hospitals (<10), the proportion of hospitals undertaking continuous surveillance in 2011/12 was highest in hip prosthesis and knee prosthesis at 55% and 52% respectively representing an increase from the previous year. For coronary arterybypassgraft(CABG)andrepairofneckoffemur,theproportionwas46%.Thiswasfollowedbyspinal surgery, reduction of long bone fracture and large bowel surgery at just over 40%. The remaining categories (vascular and abdominal hysterectomy) achieved <25%.

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Figure 1: Trends in the number of operations and participating hospitals by surveillance quarter, NHS hospitals in England

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2 .2 Data quality

Data completion for key SSI risk factors is essential for risk stratification purposes thus enabling hospitals to investigate factors underpinning unusual deviations from the SSI national benchmark.

Table 1 shows the proportion of total records submitted to the national database with completed fields for key data items by surgical category during 2011/12. Data completeness varied by data item and category.

Data completion was above 99% for age, patient sex, duration of operation, wound class, date of admission.The completion rate for antibiotic prophylaxis was high at 96% overall (not shown). Data completeness for ASA score was slightly lower at 93% overall with variation between surgical categories ranging from 75% of submitted recordsinCABGsurgeryto97%incranialsurgery.Datacompletenesswasconsiderablylowerforheightandweight used to calculate body mass index (BMI) at 42% overall, reflecting the fact that completion of these fields isoptionalforhospitals.BMIdatacompletionwashighestinCABG(73%)andcardiacnon-CABGsurgery(69%)followed by breast and knee prosthesis surgery (53% and 52% respectively).

2 .3 Patient and operation-related characteristics

Key patient and operation-related characteristics derived from surveillance records for patients undergoing surgery in 2011/12 are shown in Table 2. The median age varied by surgical category being lowest in abdominal hysterectomy (51 years) and highest in repair of neck of femur (84 years). The median duration of operation (in minutes) also varied according to surgical category, with bile duct, liver and pancreatic surgery, cardiac surgery, CABGandgastricsurgeryallhavingmediandurationsinexcessof200minutes.ThecategoriesofsurgerywiththehighestproportionofpatientswithanASAscoreof≥3wereinCABG(73%),limbamputation(71%)andvascularsurgery (70%). Large bowel surgery, small bowel surgery and limb amputation had the highest proportion of records with a wound class that was contaminated or dirty (44%, 25% and 20% respectively).

Kneeprosthesis,hipprosthesis,CABG,abdominalhysterectomy,cholecystectomyandcardiacnon-CABGsurgeryhad the highest proportion of patients classed as overweight or obese (BMI ≥ 25 kg/m2). The categories with the lowest proportion of patients that were overweight or obese were in repair of neck of femur and reduction of long bone fracture at <50%. The proportion of patients that were male varied according to category with the majority ofcardiac(CABGandcardiacnon-CABG),vascularandlimbamputationpatientsbeingmale.Theproportionofoperations performed on an emergency basis (defined as procedures that are immediate, unplanned and life-saving or those that are performed immediately after resuscitation) ranged from 0% in orthopaedic surgery to 9% in vascular surgery. The proportion of procedures that were revisions was generally low, with the highest observed in hip prosthesis (10%) and spinal surgery (8%).

The proportion of patients receiving surgical antimicrobial prophylaxis was highest (≥98%) in upper gastro-intestinal surgery (bile duct/liver/pancreatic; gastric and cholecystectomy), orthopaedic surgery (hip prosthesis, knee prosthesis and reduction of long bone fracture) and cardiac surgery. These proportions reflect the current recommendations aimed at high-risk groups[4][5]. Surgical categories with the lowest proportion of patients receiving surgical antimicrobial prophylaxis were in breast surgery and limb amputation (76%).

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Table 1: Proportion of submitted records with complete data for key data items, NHS hospitals in England, 2011/12

Total No. operations

Age ASA scoreDuration of operation

Wound class BMI Patient sexDate of

admission OPCS code

Abdominal hysterectomy 790 100.0 95.8 100.0 100.0 44.1 100.0 100.0 100.0

Bile duct, liver and pancreatic surgery 497 100.0 92.0 100.0 100.0 39.8 100.0 100.0 100.0

Breast surgery 1,631 100.0 89.9 100.0 100.0 53.0 100.0 100.0 100.0

CABG† 5,052 100.0 75.3 100.0 99.7 73.2 100.0 100.0 100.0

Cardiac(non-CABG) 2,563 100.0 58.5 100.0 100.0 68.6 99.9 100.0 100.0

Cholecystectomy 145 100.0 89.7 100.0 100.0 14.5 100.0 100.0 100.0

Cranial surgery 612 100.0 96.6 100.0 100.0 17.2 100.0 100.0 100.0

Gastric 191 100.0 93.2 100.0 100.0 - 100.0 100.0 100.0

Hip replacement 37,447 100.0 95.9 100.0 99.7 47.5 100.0 100.0 99.9

Knee replacement 38,680 100.0 96.4 100.0 99.3 52.2 100.0 100.0 99.9

Large bowel surgery 3,696 100.0 93.8 100.0 99.4 38.2 100.0 100.0 100.0

Limb amputation 390 99.5 88.7 100.0 99.5 14.6 100.0 100.0 100.0

Reduction of long bone fracture 3,293 100.0 95.5 99.4 99.9 4.4 100.0 100.0 100.0

Repair of neck of femur 16,518 100.0 90.9 100.0 99.8 11.2 100.0 100.0 100.0

Small bowel surgery 626 100.0 92.0 100.0 99.8 16.8 100.0 100.0 100.0

Spinal surgery 5,943 100.0 92.3 100.0 99.4 24.6 100.0 100.0 100.0

Vascular surgery 1,118 100.0 96.2 100.0 99.7 25.2 100.0 100.0 100.0

Total 119,192 100.0 93.3 100.0 99.6 42.2 100.0 100.0 99.9

* no BMI submitted; † coronary artery bypass graft;

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Table 2

* excluding unknown values; † coronary artery bypass graft; cells with ‘–‘ indicate data not available

Table 2: Patient and operation-related characteristics* by surgical category, NHS hospitals in England, 2011/12

Median age in years (IQR)

Median duration of operation in minutes

(IQR)ASA ≥ 3 (%)

Contaminated/ Dirty incision (%)

BMI ≥ 25 (%)** Male (%)Emergency

(%)Revision

(%)

Antibiotic prophylaxis

(%)

Abdominal hysterectomy 51 (44 - 64) 88 (65 - 120) 13.5 0.3 71.6 N/A 0.1 N/A 96.4

Bile duct, liver or pancreatic 65 (55 - 72) 230 (160 - 300) 28.9 0.6 64.7 53.5 0.4 0.2 99.8

Breast surgery 55 (46 - 67) 61 (40 - 92) 8.6 0.6 59.5 2.2 0.1 2.0 75.8

Cardiac surgery 67 (49 - 76) 226 (177 - 290) 94.5 0.8 65.6 62.0 2.6 0.4 99.4

Cholecystectomy 64 (55 - 73) 200 (120 - 287) 41.5 0.0 66.7 44.8 0.7 N/A 99.3

CABG† 68 (61 - 75) 228 (191 - 266) 96.1 0.1 74.8 79.7 1.6 0.1 99.2

Cranial surgery 58 (44 - 71) 102 (57 - 174) 50.3 1.6 65.6 57.0 2.0 0.2 99.2

Gastricsurgery 62 (49 - 73) 204 (105 - 300) 34.3 1.1 - 51.3 0.0 3.7 99.5

Hip replacement 70 (62 - 77) 85 (66 - 108) 20.8 0.2 76.4 39.5 0.3 10.1 97.8

Knee replacement 70 (63 - 77) 80 (64 - 99) 20.6 0.1 88.6 42.5 0.1 5.5 97.9

Large bowel surgery 68 (59 - 77) 165 (120 - 221) 35.9 19.7 62.5 51.4 7.4 N/A 96.1

Limb amputation 67 (55 - 75) 55 (29 - 85) 80.4 25.0 54.4 70.5 1.8 N/A 76.2

Reduction of long bone fracture 61 (37 - 78) 81 (60 - 111) 29.1 1.3 43.8 42.6 7.7 N/A 97.5

Repair of neck of femur 84 (78 - 89) 66 (51 - 87) 65.7 0.4 31.8 26.6 4.7 0.2 94.9

Small bowel surgery 64 (45 - 74) 110 (70 - 187) 40.3 43.7 54.3 53.2 5.0 0.6 95.4

Spinal surgery 54 (41 - 67) 110 (73 - 160) 20.8 0.3 67.8 49.8 0.7 7.8 98.3

Vascular surgery 72 (64 - 79) 160 (117 - 223) 72.7 0.9 59.9 70.8 8.6 0.2 92.8


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2 .4 Rate of surgical site infection by surgical category

Table 3 shows the volume of surgical procedures, the number of SSI cases and the SSI incidence by surgical category using data collected by NHS hospitals and NHS treatment centres for surgery undertaken between April 2007 and March 2012. Over this period a total of 496,373 procedures on 17 surgical categories were submitted by 224 participating hospitals representing 201 NHS Trusts. An additional 11 NHS Treatment centres participated in the surveillance.

Asthesurveillanceforbreast,cardiac(non-CABG)andcranialsurgerywasintroducedinApril2010,datafor these categories should be interpreted with caution due to small number of procedures and number of participating sites.

The cumulative incidence (%) of SSI varied between surgical categories according to the likelihood of microbial contamination at the operative site. The highest incidence (or risk) was observed in gastro-intestinal surgical procedures with large bowel surgery having the highest risk at 10.3%. The lowest risk was observed in hip and knee prosthesis surgery (0.7% and 0.6% respectively). The low incidence of SSI in hip and in knee prosthesis surgery was comparable with estimates reported by other countries in Europe[6].

Capturing readmission SSIs improves the comprehensiveness of the SSI estimate by capturing infections occurring post discharge. SSIs detected through readmission tended to account for a relatively high proportion of cases in surgical categories with a relatively short length of hospital stay such as breast, knee and hip and surgery where readmission cases accounted for 84%, 62% and 47% of all cases respectively. In categories with relatively longer lengthofhospitalstay(gastro-intestinal,repairofneckoffemur,CABGandlimbamputationsurgeries),readmissionSSIs accounted for < 25% of all SSIs detected.

To take into account the variation in the length of patient follow-up during their hospital stay, the incidence density was computed. This metric adjusts for SSIs being more likely to be detected in patients with a longer length of hospital stay. After taking into account the length of inpatient follow-up, the variation between surgical categories was less acute when compared to the inpatient-based cumulative incidence (Table3). However, large bowel surgery remains as having the highest SSI incidence whilst cranial surgery becomes the lowest.

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± from April 2010; † coronary artery bypass graft; ‡ from July 2008

Table 3: SSI incidence by surgical category, all NHS hospitals in England, April 2007 - March 2012

Surgical Site Infections

No. operations

No. hospitals

Median length of hospital stay

(days)

No. Inpatient

No. Inpatient/ readmission

Inpatient & Readmission (%)

95% CLsMedian time to infection

(days)

Incidence density/1,000 days inpatient

follow up

95% CLs

Abdominal hysterectomy 5,431 30 4 32 79 1.5 1.2 – 1.8 9 1.3 0.9 - 1.9

Bile duct, liver and pancreatic surgery 1,811 7 9 114 126 7.0 5.8 – 8.2 8 5.3 4.3 - 6.3

Breast± 3,115 12 1 5 31 1.0 0.7 – 1.4 11 0.9 0.3 – 2.0

Cholecystectomy 764 6 4 16 21 2.8 1.7 – 4.2 8 3.3 1.9 – 5.4

CABG † 27,084 21 7 858 1,158 4.3 4.0 – 4.5 11 3.4 3.2 – 3.6

Cardiac(non-CABG)± 4,143 8 8 42 55 1.3 1.0 – 1.7 10 0.8 0.6 – 1.1

Cranial± 1,171 3 5 3 14 1.2 0.7 – 2.0 19 0.3 0.1 – 0.9

Gastric 1,162 9 10 36 38 3.3 2.3 – 4.5 9 3.0 2.1 – 4.2

Hip prosthesis 161,482 195 5 617 1,164 0.7 0.7 – 0.8 14 0.6 0.5 - 0.6

Knee prosthesis 175,605 194 5 384 1,009 0.6 0.5 – 0.6 15 0.4 0.3 – 0.4

Large bowel 15,321 47 8 1,355 1,577 10.3 9.8 – 10.8 8 8.1 7.7 – 8.5

Limb amputation 2,296 21 13 83 91 4.0 3.2 – 4.8 10 2.4 1.9 – 3.0

Reduction of long bone fracture‡ 10,873 36 3 79 141 1.3 1.1 – 1.5 15 0.9 0.7 – 1.1

Repair of neck of femur‡ 56,266 126 13 702 886 1.6 1.5 – 1.7 14 0.7 0.7 – 0.8

Small bowel 3,121 17 8 198 211 6.8 5.9 – 7.7 8 5.5 4.8 – 6.3

Spinal ‡ 19,110 22 2 93 184 1.0 0.8 – 1.1 13 0.9 0.7 – 1.1

Vascular 7,618 34 6 154 210 2.8 2.4 – 3.2 12 2.1 1.8 – 2.5

Total 496,373 235 4,771 6,995

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3 Risk factors for SSI

Table 4 shows the cumulative incidence of SSI according to given risk factors for each surgical category. The risk estimates are based on available data for that risk factor.

In general the risk of SSI increased in the older age group (≥65 years) except in cardiac, bile duct/liver/pancreatic, cholecystectomy, gastric, limb amputation and vascular surgery where the reverse was observed with the risk being higher in younger patients (<65 years). Patients with ASA score of ≥3 had an increased risk except in gastric surgery where the reverse pattern was observed. Across all categories, duration of operation greater than the national 75th percentile had an increased risk of SSI compared to procedures below this threshold.

The risk of SSI increased among patients who were overweight or obese (BMI ≥25 kg/m2)inCABG,cardiacnon-CABG,breastandkneesurgery.TheBMIresultsfortheremainingsurgicalcategoriesshouldbeinterpreted with caution owing to the low level of data completion for this risk factor in these categories (Table 2).

In terms of patient sex, female patients exhibited a higher risk compared to male patients in cardiac, cranial, spinal and vascular surgery. The reverse was true in bile duct/liver/pancreatic surgery, gastric, limb amputation, reduction of long bone fracture and small bowel surgery.

Having an emergency procedure increased the risk of SSI for every surgical category. It should be noted thatinbileduct/liver/pancreaticsurgeryandcardiac(non-CABG)theunderlyingsampleforemergencyprocedure was very small hence the risk estimate for this group should be interpreted with caution. Having a revision procedure was also associated with an increased risk of SSI in each surgical category.

The results show that the risk of SSI increased consistently across all surgical categories where the wound class was contaminated/dirty or for emergency procedures.

Although some of these risk factors are non-modifiable, it is important to implement peri-operative measures to optimise SSI outcome. The NICE guideline on the prevention of SSI identifies evidence-based measures in the peri-operative phase such as glucose control, skin asepsis and antimicrobial prophylaxis to minimise the risk of wound infection[4].

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Surveillance of Surgical Site Infections in NHS hospitals in England 2011/2012Table 4: Cumulative incidence of SSI stratified by risk factor*, by surgical category, NHS hospital in England, 2011/12

Age (years) ASA scoreDuration of operation

Wound class* BMI Patient sex Surgery type Revision

<65 ≥65 <3 ≥3 <T-time ≥T-time C/C-C C/D <25 ≥25 Female Male Elective Emergency No Yes

Abdominal hysterectomy 1.2 2.2 1.2 2.0 1.5 1.1 1.4 0.0*** 0.0 2.0 1.4 N/A 1.4 0.0*** 1.4 N/A

Bile duct, liver or pancreatic 4.7 3.8 3.4 5.3 2.7 5.9 4.3 0.0*** 2.9 3.1 3.5 4.9 4.0 50.0*** 4.2 0.0***

Breast surgery 0.8 1.1 1.0 - 0.7 2.6 0.9 0.0*** 0.6 0.8 0.9 - 0.9 0.0*** 0.8 3.1

Cardiac surgery 2.1 1.0 1.2 1.1 1.4 2.0 1.5 0.0*** 0.7 1.2 2.0 1.2 1.4 6.0 1.5 10.00***

Cholecystectomy 8.0 7.1 6.6 9.3 6.5 6.6 7.6 - 14.3 21.4 7.5 7.7 7.6 0.0*** 7.6 N/A

CABG** 3.4 4.3 4.7 3.8 3.4 7.2 3.9 20.0 1.8 4.5 6.1 3.4 3.9 4.8 3.9 0.0***

Cranial surgery 1.3 1.7 1.0 1.7 1.3 3.0 1.5 0.0*** 3.2 2.7 1.9 1.2 1.5 0.0*** 1.5 0.0***

Gastricsurgery 4.6 2.4 5.1 1.6 3.8 3.7 3.7 0.0*** ‡ 2.2 5.1 3.7 - 3.8 0.0***

Hip replacement 0.6 0.7 0.5 1.2 0.5 1.1 0.6 2.6 0.6 0.6 0.6 0.7 0.6 0.0 0.5 1.4

Knee replacement 0.6 0.7 0.5 1.3 0.6 1.4 0.7 12.0 0.6 0.8 0.5 0.9 0.7 0.0*** 0.6 2.0

Large bowel surgery 11.0 11.2 9.9 14.6 9.8 13.0 9.6 17.5 9.5 14.4 11.3 11.0 10.5 18.8 11.1 N/A

Limb amputation 4.2 2.2 2.9 3.6 3.7 2.6 2.4 5.2 0.0 0.0 2.6 3.3 3.1 0.0*** 3.1 N/A

Reduction of long bone fracture 0.7 1.6 0.8 1.8 0.9 1.7 1.1 0.0*** 1.2 1.6 1.0 1.2 1.1 1.6 1.1 N/A

Repair of neck of femur 1.1 1.5 1.1 1.7 1.3 2.1 1.5 3.0 1.1 1.9 1.4 1.6 1.5 1.7 1.5 0.0***

Small bowel surgery 8.2 8.1 6.4 9.9 8.0 9.1 6.0 11.0 6.3 15.8 6.8 9.3 7.9 12.9 8.2 0.0***

Spinal surgery 1.0 1.0 0.7 2.0 0.7 2.1 1.0 0.0*** 0.9 1.3 1.2 0.8 1.0 0.0*** 1.0 1.1

Vascular surgery 5.4 1.7 2.4 3.1 2.0 3.9 2.8 0.0*** 0.9 3.6 3.4 2.5 2.7 3.1 2.8 0.0***

*C /CC= clean or clean-contaminated, C/D = contaminated or dirty; **coronary artery bypass graft;

***underlying sample ≤45 ‡ data not available; cells with ‘–‘ indicate no SSI cases

13


4 Trends in rate of SSI

4 .1 National trends

Figure 2 shows the trends in cumulative incidence of SSI in orthopaedic categories between 2004/5 and 2011/12. For each category, the inpatient trend is shown in addition to the combined inpatient and readmission trend. For hip and knee prosthesis surgery, the incidence of SSI relating to inpatient-detected cases has been in decline since 2004/5 due to the decreasing length of hospital stay. Inclusion of the readmission infections from July 2008 substantially increased the SSI risk estimates for most categories of surgery.

A trend analysis was undertaken to estimate the annual change in the risk of inpatient and readmission SSI from July 2008 to March 2012. The Risk Ratio (RR) measure is used to estimate the change in risk of SSI per one year increase.

The analysis showed that the risk of inpatient and readmission SSI increased significantly in knee prosthesis surgery during the four-year period (RR: 1.12; 95% CLs: 1.05 – 1.19); P=0.001). By contrast, the inpatient SSI trends were in decline due to decreasing length of hospital stay, indicating that increases in cases detected post-discharge were driving the increase in SSI following knee surgery. Even after the exclusion of revision procedures from the knee data the increasing trends remained significant (RR:1.12; 95% CLs: 1.04 – 1.20; P=0.002). Whilst the trends broadly coincide with the recent introduction of the national guidelines on the prevention of venous thromboembolism (VTE) for patients admitted to hospital [7], an increase in the SSI incidence in other elective orthopaedic procedures considered to be at high risk of VTE (hip prosthesis and repair of neck femur) was not observed. As such, further study is needed to verify the drivers behind the increase in surgical wound infections in knee prosthesis.

A significantly decreasing trend in the risk of SSI was found for patients undergoing hip prosthesis (RR:0.92; 95% CLs:0.87 – 0.98; P=0.006). Although there were year-on-year decreases in the incidence of SSI for repair of neck of femur, the trend analysis did not reach statistical significance. Patients undergoing repair of neck of femur tend to be elderly trauma patients with complex clinical characteristics hence infection prevention may be more challenging in this group. For reduction of long bone fracture, the observed data indicated that there were annual decreases in the incidence of SSI from 2009/10.

Figure 2: Trends in the annual cumulative incidence of SSI (%) in orthopaedic categories, with upper and lower 95% confidence intervals NHS hospitals, England

*SSI rates for 2008/09 exclude April-Jun 2008 data

14


Figure 3 displays the observed trends in the inpatient SSI incidence and the combined inpatient and readmission SSI incidence for the non-orthopaedic categories of surgery.

Vascular, spinal and limb amputation surgery showed consistent decreases in the inpatient SSI incidence although there appears to be a marginal increase in 2011/12 for vascular surgery. The decline in the median length of post-operative hospital stay in recent years for limb amputation may explain the recent decreases in the inpatient SSI incidence for this category. However the statistical analysis based on inpatient and readmission data did not show evidence of significant change for these three surgical categories.

The observed trends for large bowel surgery indicated that there was an upward trend in the rate of SSI since 2009/10 (RR: 1.12; 95% CLs:1.04 – 1.20; P=0.002). In small bowel surgery, the observed trends suggested that the inpatient and readmission SSI incidence have been increasing but the analysis did not reach statistical significance (RR: 1.14; 95% CLs: 0.98 – 1.31; P=0.088). For the other three gastro-intestinal categories, bile duct/liver and pancreatic surgery exhibited a consistent decrease over the

15


four year period which was found to be significant (RR: 0.75; 95% CLs: 0.63 – 0.89; P=0.001) while cholecystectomy showed a significant increase (RR: 2.82; 95% CLs: 1.66 – 4.77; P<0.001). The increase in cholecystectomy may be related to an increase in the cohort of patients in recent years with serious underlying conditions requiring open surgery as opposed to laparoscopic surgery, the latter being excluded from the HPA surveillance. No evidence of change was found in gastric surgery and the increase observed in the last two years of the period under study was not found to be significant.

Limb amputation did not exhibit a significant trend overall despite the increase observed between 2010/11 and 2011/12. Similarly no evidence of change was found in abdominal hysterectomy.

ForpatientsundergoingCABGsurgery,therewasnosignificanttrendoverthefour-yearperiodbutthiswas most likely to have been masked by the marked peak observed only from 2008/09 to 2009/10. Trend analysis based on the last three years showed a significant decreasing trend in SSI rates (RR:0.81; 95% CLs: 0.75 – 0.89; P<0.001).

This analysis has shown that there is considerable heterogeneity in the trends between surgical categories. The factors underpinning these trends are likely to be related to patient and peri-operative factors. Although many patient-related risk factors are non-modifiable there are several aspects in the peri-operative phase that can be optimised to minimise the risk of SSI to the patient. These include glucose control and compliance with guidelines on surgical antibiotic prophylaxis relating to the choice, frequency, dose and timing of the antimicrobial agent[4][5].

Whilst trends in infection rates were either found to be stable or decreasing for many categories of surgery, clear evidence for increasing SSI trends was found in three surgical categories: knee prosthesis, large bowel surgery and cholecystectomy. Whilst not reaching statistical significance, trends in spinal and small bowel surgery also showed increases in SSI rates. These categories require continued close monitoring as a matter of priority. An in-depth investigation into these surgical categories would also be beneficial to identify key contributory factors behind the increases and possible measures to reduce infection rates.

Figure 3: Trends in the annual cumulative incidence of SSI (%), in the voluntary surveillance categories, with upper and lower 95% confidence intervals NHS hospitals, England

16


4 .2 Hospital-level trends

Trends in the cumulative incidence of inpatient and readmission SSIs from July 2008 to January 2012 were examined by hospital and by surgical category. Hospitals with less than four quarters of surveillance data during the study period were excluded as were surgical categories with less than 10 participating hospitals. A further hospital was excluded from the knee prosthesis category as it was found to be unsuitable for statistical analysis due to the lack of infections in its underlying data in the last 14 of 15 surveillance periods submitted in the study period (leading to model convergence problems). Based on the inclusion criteria, a total of 354,366 procedures from seven surgical categories submitted by 186 hospitals were available for analysis. A univariable analysis based on a generalised linear model was used for evaluating trends.

† coronary artery bypass graft

†

(2)

(3)

(1)

(1)

(4)

(1)

(9)

(17)

(6)

(9)

(113)

(117)

(70)

(3)

(4)

(1)

(9)

(1)

(8)

(1)

(1)

(2)

(21)

(19)

(3)

(8)

0% 20% 40% 60% 80% 100%Large bowel

CABG*

Spinal

Hip prosthesis

Knee prosthesis

Repair of n

eck

of femur

Reduction of lo

ng

bone fractu

re

% of hospitals available for analysis

Significant increase No significant change Significant decrease No infections

Figure 4: Hospital-specific trends in the cumulative incidence of SSI by surgical category between July 2008 to March 2012, NHS hospitals in England (number of hospitals)

NB: statistical significance level P=0.05;*coronary artery bypass graft

17


Figure 4 shows that a high proportion of participating hospitals experienced either decreasing incidence or no change in the incidence for each surgical category during the study period; this proportion was highest in repair of neck of femur (95%) and lowest in reduction of long bone fracture (56%).

SurgicalcategorieswiththelargestproportionofhospitalswithasignificantlyincreasingtrendwereCABGand large bowel surgery. However, the overall number of hospitals was relatively small in these categories.

Although a large proportion of hospitals did not experience a significant change in their SSI incidence over this time period, this may partly reflect small sample sizes. Significant changes would also be difficult to detect in clean wound surgeries like hip and knee prosthesis since the SSI rate in these procedures tends to be inherently low. Where surveillance has resulted in hospitals achieving an irreducible minimum infection rate, these will appear as hospitals with no significant change. For example, among hospitals that did not experience a significant change in hip and knee prosthesis over the study period, 35% of hospitals undertaking surveillance in hip prosthesis and 32% of hospitals in knee prosthesis had an overall incidence that was close to the national median (between 40th and 60th percentiles). The results from this analysis illustrate the reductions in SSI rates achieved by hospitals undertaking surveillance emphasising the importance of surveillance and feedback in achieving reductions and maintaining low incidence as demonstrated elsewhere [8][9][10]. The implementation of a number of national initiatives by hospitals in England have undoubtedly also played a role, including MRSA screening and decolonisation of elective surgery patients[11][12], the SSI care bundle[13] and the NICE guideline on the prevention of SSI[4].

50.0

67.0

28.2

57.7

23.5

52.848.0

57.9

50.0

33.0

71.8

42.3

76.5

47.252.0

42.1

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Inpatient Inp/Readm Inpatient Inp/Readm Inpatient Inp/Readm Inpatient Inp/Readm

Hip replacement Knee replacement Reduction ofbone fracture

Repair of neck of femur

% t

ota

l typ

e SS

I

Deep/Organ-Space

Fig 5

Fig 6

Superficial

Deep/Organ-Space Superficial

39.646.0

35.2 36.327.1 29.4

60.065.0

60.454.0

64.8 63.772.9 70.6

40.035.0

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%


Coronary artery bypass graft

Large bowel Small bowel Spinal

% t

ota

l typ

e SS

I

Figure 5: SSI type distribution in inpatient cases compared to inpatient and readmission cases, orthopaedic surgery, NHS hospitals in England, 2011/2012

*total number of SSI:39

18


5 Characteristics of surgical site infections

5 .1 Type of SSI

Figures 5 and 6 show the SSI type distribution for the inpatient cases compared to that for the inpatient and readmission cases combined by surgical category (from April 2011 to March 2012).

Overall, information on SSI type was available for all but one of the 1,646 SSIs detected in 2011/12. However, surgical categories that had a total number of inpatient and readmission SSIs that were <50 were excluded leaving four surgical categories available for analysis.

The proportion of inpatient SSIs that were classed as superficial varied by surgical category ranging from 40% in spinal surgery to 72% in knee prosthesis surgery. The observed proportions will be affected by the differences in the length of post-operative hospital stay between categories.

Readmission SSIs are by definition more serious wound complications. Inclusion of SSIs detected on readmission has substantially increased the proportion of SSIs detected that were deep or organ-space infections, particularly for the orthopaedic categories. However, in the voluntary categories the proportion of SSIs that were deep or organ-space was broadly similar to the corresponding proportion for inpatient SSIs. This is because the length of hospital stay in these categories was long enough to capture the majority of SSIs that developed.

19


Figure 6: SSI type distribution in inpatient cases compared to inpatient and readmission cases for non-orthopaedic surgery, NHS hospitals in England, 2011/2012

50.0

67.0

28.2

57.7

23.5

52.848.0

57.9

50.0

33.0

71.8

42.3

76.5

47.252.0

42.1

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%


Hip replacement Knee replacement Reduction ofbone fracture

Repair of neck of femur

% t

ota

l typ

e SS

IDeep/Organ-Space

Fig 5

Fig 6

Superficial

Deep/Organ-Space Superficial

39.646.0

35.2 36.327.1 29.4

60.065.0

60.454.0

64.8 63.772.9 70.6

40.035.0

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%


Coronary artery bypass graft

Large bowel Small bowel Spinal

% t

ota

l typ

e SS

I

20


5 .2 Causative pathogens

Data on organisms reported as causing SSIs were available for 63% of inpatient and readmission SSI cases during 2011/12 (1,036/1,646). Overall, the proportion of cases that had polymicrobial aetiology (multiple organisms reported) was 30.3% (314/1,036). This proportion was highest in gastro-intestinal procedures ranging between 39% and 60% (although the underlying total number of cases was small except in large bowel surgery). In orthopaedic surgery polymicrobial SSIs accounted for 27.6% of (162/588). The proportion of cases that had monomicrobial aetiology was generally highest in clean surgery.

SSIs with organism data yielded 1,438 isolates in total. Figure 7 shows the overall distribution of the pathogens isolated from these clinical specimens. Overall, Enterobacteriaceae were the predominant organisms accounting for 29% of total isolates (410/1,438). The second most frequent pathogen was S. aureus which accounted for 24% of isolates (349). Of total S. aureus isolates, 18% were meticillin-resistant (MRSA); this proportion represented a further decrease from 23% observed in the previous year. Of total isolates, MRSA accounted for 4%.

28.5%

MR

SA 4

.4%

13.8%

8.3%7.0%

6.2% 6.1%

4.0%

1.5%0.4%

MSS

A 1

9.9

%

0%

5%

10%

15%

20%

25%

30%

35%

Ente

robac

teria

ceae

S. au

reus

Cogau

lase n

egat

ive

staphy

lococ

ci

Ente

roco

ccus

spp

Pseu

domon

as sp

p

Other

bac

teria

Anaer

obes

Strep

toco

ccus

spp

Fung

i

Acinet

obac

ter s

pp

% o

f to

tal S

SI is

ola

tes

total number of isolates = 1,438

NB: 'Other bacteria' - mostly comprising 'other Gram-positive organisms', 'unspecified diphtheroids' and other Gram-negative bacteria'

Figure 7: Overall distribution of organisms reported as causing SSIs (inpatient and readmission), all surgical categories, NHS hospitals, England, 2011/2012

NB:‘Otherbacteria’–mostlycomprising‘otherGram-positiveorganisms’,‘unspecifieddiphtheroids’andotherGram-negativebacteria’

21


Figure 8 shows the distribution of causative organisms by surgical category where the total number of isolates reported was at least 100 per category. S. aureus was the predominant isolate in the orthopaedic categories accounting for 33% of hip prosthesis isolates (83/254), 26% of knee prosthesis isolates (63/238) and 33% of repair of neck femur isolates (87/263). Coagulase-negative staphylococci (CoNS) were more frequent in surgery involving implants – in knee prosthesis they accounted for 26% ofisolates(63/238)and24%inCABGsurgery(29/119).EnterobacteriaceaewerethepredominantcausativeorganismsinlargebowelandCABGsurgeryaccountingfor44.8%(143/319)and31.9%ofisolates (38/119) respectively.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Hip prosthesis Knee prosthesis Repair of neck offemur

CABG Large bowel

% o

f to

tal i

sola

tes

Streptococcus spp

Other anaerobes

Other bacteria incl Fungi

Pseudomonas spp

Enterococcus spp

Coagulase-negative staphylococci

MRSA

MSSA

Enterobacteriaceae

Figure 8: Distribution of organisms reported as causing SSI* by surgical category, NHS hospitals, England, 2011/2012

*isolatesfrominpatientandreadmissionSSIs;NB:‘Otherbacteria’-mostlycomprising‘otherGram-positiveorganisms’,‘unspecifieddiphtheroids’and‘other Gram-negativebacteria’.

22


Figure 9 shows the distribution of causative organisms for the combined orthopaedic categories stratified by case ascertainment method. Meticillin-sensitive S. aureus (MSSA) was the predominant pathogen regardless of detection method; it accounted for 20.8% of inpatient-based isolates (80/384) and 27.7% of readmission-based isolates (112/404). MRSA reduced considerably in 2011/12. The second most frequent pathogens isolated among inpatient-based isolates were Enterobactericeae at 20.8% (80/384) whilst CoNS were the second most frequent pathogens isolated among readmission isolates at 21.3% (86/404). Although it is possible that some of the CoNS organisms reported may be contaminants, their frequent appearance in readmission isolates is indicative of wound healing complications in relation to implanted joint prosthesis.

20.8%19.6%

17.4%

21.3%

11.5%

8.2%9.6%

3.0%

6.5%7.4%

4.4%

0.7%

2.9%

5.4%

MSS

A 2

0.8%

MSS

A 2

7.7%

0%

5%

10%

15%

20%

25%

30%

35%

40%

S. au

reus

Ente

robac

teria

ceae

Coagula

se-n

egat

ive

staphy

lococ

ci

Ente

roco

ccus

spp

Pseu

domon

as sp

p

Other

bac

teria

incl. F

ungi

Other

anae

robes

Strep

toco

ccus

spp

MRS

A 6

.7%

NB: 'Other bacteria' in orthopaedic categories - mostly comprising isolates reported as 'unspecified diphtheroids', Corynebacte-rium spp. and 'other Gram-positive organisms'.

inpatient isolates (n=384)

readmission isolates (n=404)

MRS

A 6

.0%

Figure 9: Organisms reported as causing SSI* in orthopaedic categories by detection method, NHS hospitals, England, 2011/12

NB: ‘Other bacteria’ in orthopaedic categories – mostly comprising isolates reported as ‘unspecified diphtheroids’, Corynebacterium spp. and ‘other Gram-positiveorganisms’.

23


6 Variation in rates of SSI between NHS hospitals

6 .1 Box and whisker plots

Figure 10 is a box and whisker plot showing the distribution of individual hospital rates against five percentiles (10th, 25th, 50th, 75th and 90th) by surgical category with each percentile representing a proportion of the total observations.

The box plots show inter-category variation and within each category inter-hospital variation. SSI estimates (%) that lie at extreme ends (above the 90th percentile or below the 10th percentile) may be indicative of a problem. However, when making comparisons between hospitals it is important to take into account the underlying volume of procedures: the greater the number of procedures on which the SSI estimate is based, the more precise the estimate.

% o

pera

tion

s in

fect

ed

Abdo

min

al hy

stere

ctom

y

Bile

duc

t, liv

er o

r pan

crea

tic su

rger

y

Coro

nary

Arte

ry B

ypas

s Gra

ftGa

stric

surg

ery

Tota

l hip

pro

sthes

isTo

tal K

nee

pros

thsis

Larg

e bo

wel su

rger

yLim

b am

puta

tion

Open

redu

ctio

n of

frac

ture

(up

to Ju

n 08

)Sm

all b

owel

surg

ery

Vasc

ular

surg

ery

Hip

hem

iarth

ropl

asty

(up

to Ju

n 08

)

Redu

ctio

n of

long

bon

e fra

ctur

e (fr

om Ju

l 08)

Repa

ir of

nec

k of f

emur

(fro

m Ju

l 08)

Spin

al su

rger

y (fro

m Ju

l 08)

Card

iac n

on-C

ABG

(from

Apr

201

0)

Cran

ial (f

rom

Apr

201

0)

Brea

st su

rger

y (fro

m A

pr 2

010)

0

5

10

25

20

25Percentiles

90th75th 50th

25th

10th

Figure 10: Distribution of the SSI incidence (%) for each hospital by surgical category, NHS hospitals in England, April 2007 to March 2012

* hospitals with less than 100 operations for hip, knee or abdominal surgery are excluded; for the remaining categories, hospitals with less than 50 are excluded. For categories where the number of participants <20, no box plot is presented and instead the distribution of the individual rates at hospital level are shown.

24


6 .2 Funnel plots

In 2011/12 data on 95,938 orthopaedic procedures were submitted. Of this total, 89,414 procedures were submitted by 186 hospitals representing 148 participating NHS Trusts. The remaining data (6,524 procedures) were submitted by an additional 10 independent sector NHS treatment centres. However in this year, two NHS Trusts did not contribute orthopaedic data as part of the mandatory requirement. Of 148 Trusts doing the mandatory orthopaedic surveillance in 2011/12, only 21 participated in the minimum surveillance required by the Department of Health, with the remaining 127 Trusts (86%) opting to participate more fully in surveillance of SSI by undertaking surveillance in additional orthopaedic categories or for a longer duration than required.

Figure 11 shows funnel plots displaying variation in the Trust SSI estimates based on inpatient and readmission infections by orthopaedic category in 2011/12. The cumulative incidence of SSI per 100 operations is plotted against the number of procedures for each participating NHS site. The 95% control limits (dashed black lines) define the ‘limits’ of acceptable variation in the Trust estimates so that any data point lying outside these limits is considered an outlier. These outliers are described as having ‘special cause variation’ and would require further investigation. The margin of error associated with 95% control limits is 5% meaning that there is a 1 in 20 chance that the observed Trust estimate is due to chance alone in that year. The 99% control limits (solid red lines) also define the limits of acceptable variation. However, since they are still based on the same ‘sample size’ for each Trust the confidence range is wider but this is offset with a lower margin of error (1%). The 99% control limits are presented for information since the conventional level of significance is 5%.

The funnel plots do not adjust for case-mix so the results should be interpreted with caution. These graphical displays should be used as triggers for further investigation.

The funnel plots identified a total of 12 individual NHS Trusts that were statistical outliers; of these three were high outliers and 9 were low outliers. Three Trusts were low outliers in more than one surgical category. Five Trusts were outliers in the previous year; of those one was a repeat high outlier and four were repeat low outliers indicating that their estimates were not likely to be due to chance hence indicative of systematic differences requiring further investigation. No outliers were identified in reduction of long bone fracture.

Trusts that were identified as low outliers (below the lower 95% control limit) require further investigation too. Although a low SSI incidence may reflect a high standard of infection control practice, other factors should be considered to eliminate methodological problems. These Trusts have already been contacted to make them aware of their outlier status and are encouraged to review their case ascertainment methodology. The SSI estimates for 2011/12 by NHS Trust and orthopaedic category are published as an accompanying supplement:

http://www.hpa.org.uk/webc/HPAwebFile/HPAweb_C/1317137170118

25


02

46

8C

umul

ativ

e in

cide

nce(

%)

0 500 1000 1500number of operations

Hip prosthesis 2011/12

02

46

8C

umul

ativ

e in

cide

nce(

%)

0 500 1000 1500number of operations

Knee prosthesis 2011/12

05

1015

2025

Cum

ulat

ive

inci

denc

e(%

)

0 200 400 600 800number of operations

Reduction of long bone fracture 2011/120

510

15C

umul

ativ

e in

cide

nce(

%)

0 200 400 600 800number of operations

Repair of neck of femur 2011/12

95% CI99% CITrust

95% CI99% CITrust

95% CI99% CITrust

95% CI99% CITrust

Figure 11: Cumulative incidence of SSI plotted against the number of operations by NHS Trust by mandatory orthopaedic surgical category in England, 2011/2012

26


7 Conclusions

This report summarises data submitted by hospitals participating in the HPA surgical site infection scheme. Participation in the scheme continued to increase in 2011/12 in hip and knee prosthesis. Hospitals are also able to select categories of surgery to monitor from the 17 currently on offer by the HPA programme according to local needs. Expansion of the current national system to include caesarean-section is an option as it could provide additional patient benefit given the high incidence of SSI and opportunities for prevention[14][15].

The cumulative incidence of SSI based on the last five years of data captured varied according to the operative site with the highest observed in large bowel surgery at 10.3% and the lowest in knee prosthesis surgery at 0.6%. The low incidence of SSI in hip and in knee prosthesis surgery was comparable with estimates reported by other countries in Europe[6]. Whilst the rates of infection following orthopaedic surgery were generally low, the impact of these infections can be devastating for patients as well as incurring additional hospital costs, factors which must be borne in mind in considering surveillance priorities along with opportunities for prevention.

Capturing readmission SSIs improves the accuracy of the SSI estimate. Readmission SSIs tended to account for a high proportion of total inpatient and readmission cases in surgical categories with a relatively short median length of hospital stay such as in hip and knee surgery. This clearly demonstrates the value of post-discharge surveillance. SSIs detected on readmission are more likely to be serious making their inclusion all the more important.

The trends in the inpatient/readmission incidence at national level indicated a significant decrease inSSIfollowinghipprosthesisandCABGsurgery.However,asignificantincreasewasfoundinkneeprosthesis, large bowel surgery and cholecystectomy. The increase in cholecystectomy may be related to an increase in the cohort of patients in recent years with serious underlying conditions requiring opensurgery.Givenchangesinhospitalparticipationovertimeandthepotentialinfluencethishasonnational rates, hospital-specific trends have been included in this year’s report to provide an alternative means of assessing temporal changes in infection rates. These analyses identified that the majority of hospitals had stable or decreasing rates of SSI across all categories examined over this period. Whilst the hospital-specific trends appear encouraging, the sensitivity of these analyses to detect significant changes over time will be reduced owing to small volumes of surgery at hospital level. As such, drivers for the increasing trend in infection following knee and large bowel surgery observed at national level warrant further investigation and continued close monitoring.

Comparison of mandatory orthopaedic SSI data between Trusts continued to show variation in rates between Trusts emphasising the importance of continued monitoring and bechmarking. Twelve individual NHS Trusts were identified as outliers in 2011/12 for mandatory orthopaedic surveillance. Of these, three were high outliers and 9 were low outliers. Five Trusts were outliers in the previous year indicating that their estimates were not likely to be due to chance but indicative of true deviation from the national benchmark, requiring further local investigation.

Enterobacteriaceae remain the predominant causative organisms accounting for 29% of total isolates followed by S. aureus which accounted for 24% of isolates. Of total S. aureus isolates, 18% were meticillin-resistant (MRSA), a further decrease from the previous year (23%). MRSA accounted for 4% of

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total isolates, a further decrease compared with previous years[16][17][18]. The decreasing occurrence of MRSA in SSI isolates could be partly explained by the impact of various national policies directed at controlling MRSA infection in hospitals, in particular preadmission MRSA screening[11].

Our findings continue to illustrate that the process of surveillance and feedback is key for achieving reductions and maintaining low incidence. Implementation of key national guidelines provides the means of achieving a patient safety culture within surgery.

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8Glossary

Cumulative Incidence

The total number of surgical site infections as a proportion of the total number of patients undergoing a procedure in the same category of surgery expressed as a percentage.

Incidence Density

The total number of surgical site infections (identified through inpatient surveillance) divided by the total number of days of patient follow-up. This is expressed as the number of SSIs per 1,000 days of inpatient follow up.

Confidence and Control Limits (CL)

The 95% confidence limits provide a guide to the precision of the estimate based on the number of procedures.Giventhelevelofconfidence,theyindicatethatthe‘true’incidence’couldlieanywherebetween the lower and higher confidence limits. Control limits (CL) are equivalent to exact binomial confidence limits at 95% (warning) and 99% (action). The probability of a cumulative incidence lying above the 95% CL by chance alone is less than 5%; if above the 99% CLs, this is less than 1%.

P-value

This measures the probability of an estimate (e.g. a risk) being as or more extreme than that observed in a study population occurring by chance alone if the null hypothesis is true (the assumption that there is no difference between different groups). The significance level for the P-value is conventionally set at <0.05. This means that we are prepared to accept no more than a 5% chance of being wrong in claiming that our observed result is true. If the P-value computed around an estimate is >0.05, this indicates that our data provides us with sufficient evidence to reject the null hypothesis.

Risk Ratio

This is the risk of an event e.g. an infection in an ‘exposed’ group of subjects compared to another group (‘non-exposed’). A risk ratio of one indicates that there is no difference in risk between the two groups. A risk ratio of >1 indicates that the risk is greater in the ‘exposed’ group compared to the ‘non-exposed’. A risk ratio of <1 indicates that risk is higher in the ‘non-exposed’.

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For information or queries relating to this report please contact the Department of Healthcare Associated Infections and Antimicrobial Resistance at: [email protected]

December 2012© Health Protection AgencyHPAGatewayreference:HPA12-07

surveillance of surgical site infections in nhs hospitals in england

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