the standardized infection ratio: what, why and...
TRANSCRIPT
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The Standardized Infection Ratio: What, Why and When
Jonathan R. Edwards, Mstat
Maggie Dudeck, MPH, CPH
Division of Healthcare Quality Promotion
CDC
APIC Webinar
January 26, 2011
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Objectives
● Describe the standardized infection ratio (SIR)
● Discuss business cases for the SIR by federal
and state agencies
● Explain the current approach for calculating the
SIR for two specific healthcare-associated
infection (HAI) events:
– Central line-associated BSI (CLABSI)
– Risk model methodology used for SSI SIRs
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Standardized Infection Ratio
● Standardized Infection Ratio, SIR, is a
summary measure used to compare the
HAI experience among one or more
groups of patients to that of a standard
population’s (e.g. NHSN)
● Indirect standardization method
● Accounts for differences in incidence of
HAI within the groups
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● To calculate O, sum the number of
HAIs among a reporting entity
● To calculate E, requires the use of the
appropriate aggregate data from a
standard population
SIR = Observed (O) HAIs
Expected (E) HAIs
Calculating an SIR
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Business Cases for the SIR
● Development of a national action plan to prevent
healthcare-associated infections beginning in
2008 sponsored by the Department of Health and
Humans Services (a.k.a. HHS Action Plan)
● State health departments with legislation requiring
HAI reporting for all their eligible hospitals
● CMS Inpatient Prospective Payment System
– CLABSI reporting beginning January 2011
– SSI reporting beginning January 2012
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Central Line-
associated
Bloodstream
Infections
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Edwards JR et al. Am J Infect Control 2009;37:783-805.
Annual NHSN Report
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Type of
Location
#
CLABSI
# Central
line-days
CLABSI
Rate
NHSN
Ratep-value Expected
# of
CLABSI
Medical cardiac 2 380 5.26 2.0 0.09 0.76
Medical 1 257 3.89 2.6 0.15 0.67
Med/Surgical 3 627 4.78 1.5 0.11 0.94
Neurosurgical 2 712 2.81 2.5 0.32 1.78
Total 8 1976 4.05 --- --- 4.15
Example CLABSI Rate Table
Expected Number = 380 * (2.0 / 1000) = 380 * 0.002 = 0.76
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Type of
Location
#
CLABSI
# Central
line-days
CLABSI
Rate
NHSN
Ratep-value Expected
# of
CLABSI
Medical cardiac 2 380 5.26 2.0 0.09 0.76
Medical 1 257 3.89 2.6 0.15 0.67
Med/Surgical 3 627 4.78 1.5 0.11 0.94
Neurosurgical 2 712 2.81 2.5 0.32 1.78
Total 8 1976 4.05 --- --- 4.15
Example CLABSI Rate Table
Observed 8 Standardized Infection Ratio (SIR) = = = 1.93
Expected 4.15
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External Comparison of an SIR
to its Nominal Value of 1.0
SIR = 1.0 Observed = Expected
Is the SIR of Hosp A (1.93) significantly greater
than 1.0?
Answer: p-value = 0.06 (Poisson test)
Conclusion: NO, the observed number of
CLABSI is not "significantly greater" than
expected
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State HAI Summary Report
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State CLABSI SIRs
95% CI for SIR Graphic Representation
State
No. of
Facilities
Reporting SIR Lower Upper 0 1.0 2.0
A 50 0.68 0.52 0.87
B 30 0.94 0.72 1.19
C 8 0.59 0.36 0.91
D 140 0.90 0.80 1.01
E 48 1.30 1.14 1.48
http://www.cdc.gov/hai/statesummary.html
♦
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State HAI Summary Report
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Obtaining SIRs From NHSN:
CLABSI
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Example: CLABSI Standardized Infection Ratio
You have been asked to provide a summary measure
for all 2009 CLABSIs that can be presented to the
administrators at your organization.
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CLABSI SIR Reports in NHSN
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CLABSI SIR Reports in NHSN
Output for both in-plan and all data SIR reports will include 4 tables
SIR reports can be modified similarly to rate tables:
Selection criteria (OrgID, CDC location types, etc)
Time period selection
Time unit variable selection (month, quarter, half year, year)
When # expected <1, can look at longer time period to get SIR by year, for example
CLABSI rates should continue to be an important tool in prevention efforts - SIR provides summary measure adjusting for locations within facility
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CLABSI SIR Reports in NHSNTable 1: Overall SIR for Facility
Represents an overall CLABSI SIR for those locations for which CLABSIs were monitored in your facility during the specified time period.
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CLABSI SIR Reports in NHSN Table 2: SIR by Location Type
Provides SIRs by location group and time period(s) specified.
All locations would be defined by one of these groups.
ICU-OTHER includes ICUs and inpatient wards (i.e., non-NICU/SCA locations)
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CLABSI SIR Reports in NHSNTable 3: SIR by CDC Location
Provides SIRs by CDC Location and time period(s) specified.
Example, if your facility reported CLABSI data for 2 Medical ICUs, the SIR for this CDC location would include both of those locations.
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CLABSI SIR Reports in NHSNTable 4: By Individual Location
Most granular level of SIR analysis reports
Provides SIR for each individual location and time period(s)
Months variable allows user to see how many months of summary data are contributing to SIR (similar to rate table)
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Interpretation: Overall CLABSI SIR for Facility
74 CLABSIs in 10,065 CL days were reported by this facility for first half of 2009
Based on aggregate data, 26.606 CLABSIs were expected
SIR is 2.78 – almost 3 times more CLABSIs than expected
P value and 95% CI indicate this is statistically significant
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Surgical Site
Infections
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NHSN SSI Rates
* Based on 2009 NHSN Report– Am J Infect Control 2009;37:783-805
Procedure
Code DescriptionRisk
Category SSI Rate*
VHYS Vaginal hysterectomy 0 0.73
1,2,3 1.16
CSEC Cesarean section 0 1.46
1 2.43
2,3 3.82
CBGB Coronary bypass with chest and donor incisions
0 0.35
1 2.55
2 4.26
3 8.49
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Improved Risk Adjustment
● Risk index relies on three risk factors only
Allow all available factors to be considered
● These same risk factors must differentiate risk for all types of procedures
Allow the set of risk factors to be procedure-specific
● The relative contribution of these factors are constrained to be equal
Allow each factor’s contribution to vary according to it’s significant association with risk
● What can be done to improve risk adjustment?
Build logistic regression models
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Available NHSN Risk Factors
For All Procedures General anesthesia Age
Wound class Emergency Gender
ASA score Trauma Endoscope
Duration of procedure Bed size∆ Med School Affiliation∆
For C-section Duration of labor
Weight Height Estimated blood loss
For Spinal fusion Diabetes Mellitus
Spinal level Approach/Technique
For Hip/Knee prosthesis
Total/Partial Primary/Revision
∆Hospital-level factor
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Logistic Regression AnalysisSSI after VHYS (N=19,012)*
Factor Parameter Estimate OR p-value
Intercept -5.448 - -
Age (≤44 vs >44) 0.520 1.659 <0.0001
ASA (3/4/5 vs 1/2) 0.425 1.529 0.0415
Duration (>100 vs ≤100) 0.501 1.650 0.0019
Med school affiliation (Y vs N) 1.069 2.912 <0.0001
* Unpublished NHSN data, 2006-2008
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Logistic Model
for VHYS:
Factor Parameter Estimate
Intercept -5.448
Age (≤44 ) 0.500
ASA (3/4/5 ) 0.425
Duration (>100 min) 0.501
Med school affiliation (Y) 1.069
logit ( ) = + 1 X1+ 2 X2 + 3 X3 + 4 X4
= -5.448 + 0.500 (Age ≤ 44* ) +
0.425 (ASA 3/4/5* ) +
0.501 (Duration >100* ) +
1.069 (Med school affiliation* )
*For these risk factors, if present = 1; if not = 0
p̂
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Example VHYS Patient #1Risk Factors
● Age = 40
● ASA score = 4
● Duration = 117 min
● Med school affiliation = Y
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Logistic ModelCalculation for Example VHYS Patient #1
logit( ) = + 1 X1+ 2 X2 + 3 X3 + 4 X4
logit( ) = -5.448 + 0.500 (1) + 0.425 (1) + 0.501 (1) + 1.069 (1) = -2.953
Solve for : = e logit ( ) / (1 + e logit ( ))
= e -2.953 / (1 + e -2.953)= 0.050 or 5.0% riskp̂
p̂
p̂ p̂ p̂ p̂
p̂
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List of Patient Risk FactorsSSI after VHYS
Total O = 3 E = 0.85 E = 2.91
Patient Age Duration
ASA
Score
Med School
Affiliation SSI Risk Index Model
1 40 117 4 Y 0 0.012 0.050
2 53 95 2 N 0 0.007 0.004
3 30 107 2 Y 1 0.007 0.033
. . . . . . . .
. . . . . . . .
. . . . . . . .
100 37 128 4 Y 1 0.012 0.050
p̂Prob of SSI ( )
Standardized Infection Ratio (SIR) = 3 / 0.85 = 3.5 3 / 2.91= 1.0
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List of Patient Risk FactorsSSI after CSEC
Risk Index Model
1 2 60 CC Y Y 25 277 Y 31 2 0 0.024 0.064
2 4 95 CO Y Y 22 321 N 32 0 1 0.038 0.158
3 2 50 C N N 31 179 N 40 1 0 0.015 0.018
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
100 2 28 4 Y Y 34 219 Y 30 3 1 0.024 0.050
Total O = 4 E = 1.52 E = 4.52
p̂Prob of SSI ( )
SIR = 4 / 1.52 = 2.6 4 / 4.52= 0.9
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Obtaining SIRs From NHSN:
SSI
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Example: SSI Standardized Infection Ratio (SIR)
You have been asked to provide a summary measure
for all 2009 surgical site infections that can be
presented to the administrators at your organization.
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SSI SIR
Can obtain SIRs at most granular level to overall,
“big picture” level
Can obtain Surgeon-specific SIRs
Can obtain “All SSI” SIRs or “Complex A/R” SIRs
Includes:
All SSI SIR Includes Superficial, Deep & Organ/Space
Includes Primary only
Includes Admit, Readmit & Post discharge
Complex A/R Includes only SSIs identified on Admit/Readmit
Includes only inpatient procedures
Includes only Deep & Organ/Space
Includes Primary Deep incisional
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SSI SIR
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SSI SIRTable 1: Overall SSI SIR for Facility
Overall SIR for those operative procedures for which SSI were monitored in your facility during the specified time period.
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SSI SIRTable 2: SSI SIRs by NHSN Operative Procedure
SIR by operative procedure category and time period specified.
In this “All SSI SIR” table 2, inpatient and outpatient procedures are grouped together.
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SSI SIRTable 3: SSI SIRs by NHSN Operative Procedure & Setting
SIR by NHSN operative procedure category and outpatient status, as well as time period.
This table will only be available in the “All SSI SIR” output options.
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SSI SIRTable 4: Incomplete Procedures
Provides a count of the procedures and SSIs that were excluded from the SIRs.
Will only list those procedures that were excluded due to missing risk factors, or records that were recorded with outliers, as well as custom procedures.
Next step: Review the “Line Listing –
Incomplete Procedures for SSI SIR”
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SSI SIRLine List – Incomplete Procedures
Recommend modifying
this line list to limit to the
procedure, time period,
and risk factors in
question.
Note that procedures will
be excluded from the
SIR if they meet one or
more of the defined
exclusion criteria
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SSI SIRLine List – Incomplete Procedures
SSI SIR exclusion criteria*:
– Missing one or more of the model’s risk factors
– Procedure duration is <5 minutes or >IQR5• IQR5: 5 times the interquartile range above the 75th percentile. EXAMPLE:
the interquartile range (Q3-Q1) = 30 minutes and the 75th percentile is 100
minutes; the IQR5 would be 100+(5*30) = 250 minutes
– Procedure date is ≤ patient’s DOB
– Patient’s age at procedure is ≥ 109 years
– Wound class (swClass) = ‘U’
– Approach = ‘N’ (for FUSN and RFUSN only)
– Spinal Level = ‘N’ (for FUSN only)
*Full details can be found in the SIR Newsletter: http://www.cdc.gov/nhsn/newletters.html
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SSI SIRLine List – Incomplete Procedures
After reviewing this line list, the following steps are
recommended:
1. Obtain missing information (in this example, obtain ASA).
2. Edit the procedure record in NHSN to include this
information.
3. Re-generate datasets.
4. Run the desired SSI SIR output option.
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SSI SIRInterpretation
During 2009, there were 425 procedures performed and 6 SSIs
identified.
Based on the NHSN 2006-2008 baseline data, 4.653 SSIs were
expected.
This results in an SIR of 1.29 (6/4.653), signifying that during this
time period our facility identified 29% more SSIs than expected.
The p-value and 95% Confidence Interval indicate that the number
of observed SSIs is not significantly higher than the number of
expected SSIs.
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Measuring Progress
Group Baseline SIR Follow-up SIR Comparison
A 1.2 1.3 Same
B 0.9 0.8 Same
C 2.7 1.9 Different Lower
D 0.7 1.1 Different Higher
E 1.3 1.1 Different Lower
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● Risk-adjustment incorporated in the SIR is limited
primarily by data collection burden
● The SIR summary measures could mask
differences in HAI experience within the strata
being summarized
● Differences in patient composition could explain
some of the variation when comparing SIRs
Limitations
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● Standardized Infection Ratio (SIR) is a useful
metric for summarizing HAI experience
● Procedure-specific multivariate models are an
improvement over simple stratification methods
– Better estimate and differentiate SSI risk
● Longitudinal comparisons using the SIR will help to
measure prevention of HAI
Summary
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Where can I find more information about the SIR?
NHSN Newsletter
Special Edition: Your
Guide to the
Standardized Infection
Ratio (SIR)
Includes CLABSI and
SSI information and
interpretations
Examples of output
Predictive Risk Factors
for SSI SIR modelshttp://www.cdc.gov/nhsn/newletters.html