assessment of blood pressure in patients with type 2 diabetes: comparison between home blood...

Assessment of blood pressure in patients with Type 2diabetes: comparison between home blood pressuremonitoring, clinic blood pressure measurement and24-h ambulatory blood pressure monitoring

M. G. Masding, J. R. Jones, E. Bartley and D. D. Sandeman

Abstract

Aims To compare a home blood pressure (BP) monitoring device and clinic

BP measurement with 24-h ambulatory BP monitoring in patients with Type 2

diabetes mellitus (DM).

Methods Fifty-®ve patients with type 2 DM had BP measured at three

consecutive visits to the DM clinic by nurses using a stethoscope and mercury

sphygmomanometer (CBP). Twenty-four-hour ambulatory BP was measured

using a Spacelabs 90207 automatic cuff-oscillometric device (ABPM). Subjects

were then instructed in how to use a Boots HEM 732B semiautomatic cuff-

oscillometric home BP monitoring device and measured BP at home on three

speci®ed occasions on each of 4 consecutive days at varying times (HBPM).

Results Correlations between HBPM and ABPM were r = 0.88, P < 0.001

for systolic BP and r = 0.76, P < 0.001 for diastolic BP, with correlations

between CBP and ABPM being systolic r = 0.59, P < 0.001, diastolic r = 0.47,

P < 0.001. HBPM agreed with ABPM more closely compared with CBP (CBP

+10.9/+3.8 (95% con®dence intervals (CI) 6.9, 14.8/1.6, 6.1) vs. HBPM +8.2/

+3.7 (95% CI 6.0, 10.3/2.0, 5.4)). The sensitivity, speci®city and positive

predictive value of HBPM in detecting hypertension were 100%, 79% and

90%, respectively, compared with CBP (85%, 46% and 58%, respectively).

Conclusions In patients with Type 2 DM, home BP monitoring is superior to

clinic BP measurement, when compared with 24-h ambulatory BP, and allows

better detection of hypertension. It would be a rational addition to the annual

review process.

Diabet. Med. 18, 431±437 (2001)

Keywords type 2 diabetes, blood pressure, blood pressure measurement

Abbreviations BP, blood pressure; DM, diabetes mellitus; HBPM, home blood

pressure monitoring; CBP, clinic blood pressure measurement; ABPM, 24-h

ambulatory blood pressure monitoring

Introduction

The pivotal role of hypertension in the development of

complications of diabetes was highlighted by the United

Kingdom Prospective Diabetes Study (UKPDS) [1].

Correspondence to: Dr Mike Masding, Research Registrar, The Endocrine

Unit, F Level, West Wing, Southampton General Hospital, Tremona Road,

Southampton SO16 6YD, UK. E-mail: [email protected]

Department of Diabetes and Endocrinology,

Southampton University Hospitals NHS Trust,

Southampton, UK

Accepted 3 February 2001

ã 2001 Diabetes UK. Diabetic Medicine, 18, 431±437 431

Knowledge of true blood pressure (BP) is therefore critical

to the care of patients with Type 2 diabetes (DM). In

clinical practice performing a resting BP during the routine

visit makes an estimate of true BP. Whilst BP measured by

this method is an important risk factor for cardiovascular

disease (CVD) in populations, its predictive value in

individual patients is poor [2]. The problem is greater for

those caring for patients with DM. The UKPDS has set

tighter goals for treatment (a target BP of 140/80 mmHg),

thus any `white-coat' effect will push patients with

normotension into the treatment group. The routine

practice of bringing patients back for repeat BP screening

and then a further clinic appointment for decision making

is inef®cient and there is need for an alternative strategy.

The current gold standard for measuring 24-h BP is

ambulatory monitoring (ABPM) over 24 h. This, however,

is expensive and cannot be offered to a large population

group such as patients with Type 2 DM. Previous studies

[3±6] in non-diabetic populations have been inconclusive

as to whether patient BP self-monitoring at home (HBPM)

is an effective strategy. In this study we have examined the

potential role of patient HBPM prior to the annual review

clinic, comparing it with ABPM and clinic BP (CBP)

assessment.

Patients and methods

Ethical approval was obtained from the local ethics committee.

Consecutive patients with Type 2 DM (regardless of treatment)

aged < 75 years and attending the routine diabetes clinic were

invited to join the study. Type 2 DM was de®ned by the

requirement for treatment with either diet or oral hypoglycae-

mics for at least 1 year after diagnosis. Subjects were only

excluded if they were unable or unwilling to give consent to enter

the study. During the study period no changes were made to

their treatment for either diabetes or hypertension.

During the clinic visit, their BP was measured by the clinic

nurse, which is the usual practice at our clinic and also at the

General Practitioners who participate in the local shared care

scheme (nurses were instructed to measure BP to the nearest

2 mmHg using a mercury sphygmomanometer with an appro-

priately sized cuff and stethoscope, after the patient has been

seated for 5 min). The CBP measurement for each patient is the

mean of three measurements taken over 6 months at different

visits to the diabetes clinic at both the hospital and in general

practice. Whilst the nurses were briefed prior to the study on the

importance of hypertension and the need for accurate results,

they were not informed of the study itself.

After recruitment, subjects were ®tted with a previously

validated [7] Spacelabs 90207 automatic cuff-oscillometric

device to assess ABPM. The BP for each patient was measured at

half-hourly intervals during the daytime, and hourly during the

night. The mean BP reported is the average of all BP readings

throughout the 24 h, whilst the daytime and night-time values

are the means of readings between 06.00 h and 17.00, and 17.00

and 06.00 h, respectively.

Following the completion of 24-h ambulatory monitoring,

subjects were instructed how to use a Boots HEM 732B

semiautomatic cuff-oscillometric home BP monitoring device,

with an appropriately sized cuff. These devices are accurate to

6 4 mmHg (Boots PLC, personal correspondence). We valid-

ated the devices by measuring BP in ®ve healthy subjects using

the Boots HEM 732B device at the same time as the Spacelabs

90207 machine, to exclude systematic bias in the study. BP was

measured in the same and different arms on 10 different

occasions on each healthy subject to assess whether using the

right or left arm would affect the study. The bias for using the

HBPM and ABPM devices at the same time was 2.0/

+0.1 mmHg, whilst for different arms the bias was +1.6/

+2.7 mmHg, indicating there was an acceptable agreement

between the devices.

The subjects were instructed to check their BP at three speci®c

times on each of 4 consecutive days; these times were spread

throughout the day (08.00, 13.00, 17.00, 20.00, 23.00 h)

according to a prede®ned protocol. The mean BP, daytime and

night-time BP were calculated from these values (mean, all

values included: daytime, 08.00, 13.00, 17.00 h; night-time,

20.00, 23.00 h).

We prede®ned ranges for BP differences which would be

acceptable in clinical practice, at 610/5 mmHg. Using the

recently published British Hypertension Society guidelines for

hypertension management [8], we used the recommended audit

standard as a `cut-off' for hypertension (< 140/85 for CBP,

< 140/80 for HBPM and ABPM) and the sensitivity and

speci®city of CBP and mean HBPM values for detecting

hypertension were calculated. It has previously been estimated

that night-time BP should fall by 20% [9]. In order to assess

whether HBPM could be used to detect loss of night-time BP

variation, we compared the HBPM reading at 13.00 h with

HBPM at 23.00 h over the 3 days, looking for a fall of 20/

10 mmHg. Mean ABPM between 00.00 and 06.00 was then

assessed looking for a fall in BP of 20/10 mmHg, compared with

the mean day-time (09.00±17.00) ABPM. In both HBPM and

ABPM, if there was no such fall, the patients were classed as

`non-dippers'. Using measurement of ABPM as the gold stand-

ard for detecting `non-dippers', we calculated the sensitivity and

speci®city of comparing HBPM at 13.00 and 23.00 in detecting

these patients. The ABPM and HBPM daytime:night-time BP

ratios (ABPM: mean daytime BP/mean night-time BP; HBPM:

BP at 13.00/BP at 23.00) were also compared. HBPM measure-

ments on the days 1 and 4 of the study were compared to assess

whether there was a stress effect of measuring BP at home on the

®rst day.

Statistical analysis

CBP and HBPM values were compared with the `gold standard'

of ABPM using Bland and Altman's method [10], and using

correlation coef®cients obtained by the use of Statistical Package

for Social Sciences (SPSS) for Windows version 6.0. We took the

modulus of the difference between the CBP and ABPM values

and contrasted it with the corresponding differences between

HBPM and ABPM (for systolic and diastolic BP). Mean

differences and 95% CIs were calculated from a paired t-test

432 Blood pressure assessment in Type 2 diabetes · M. G. Masding et al.

ã 2001 Diabetes UK. Diabetic Medicine, 18, 431±437

procedure and P-values from a Wilcoxon signed ranks test from

SPSS.

Results

Patient characteristics

Fifty-eight patients were recruited into the studyÐthree

subjects dropped out (one because the home BP monitor

cuff was too narrow, one who refused to monitor BP at

home and one who could not operate the home BP

monitor). The characteristics of the 55 subjects included

in the analysis and their treatments for hypertension and

Type 2 diabetes are shown in Table 1.

Blood pressure readings

Mean (standard deviation (SD)) BP as assessed by ABPM

was 141/81 (19/11). The estimate of BP from CBP was 152/

85 (20/11) and from HBPM 149/84 (16/9). With respect to

systolic BP, the modulus of the differences between CBP

and ABPM were on average 4.5 mmHg (95% CI 1.4, 7.7)

greater than the modulus of the differences between HBPM

and ABPM (Wilcoxon signed rank test, P = 0.012). The

corresponding contrast for diastolic BP measurement

showed the differences between CBP and HBPM were

not so marked, the difference between CBP and ABPM

being on average 1.7 mmHg (95% CI 0.1, 3.4) greater than

the difference between HBPM and ABPM (Wilcoxon

signed rank test, P = 0.108). Correlations between HBPM

and ABPM were r = 0.88 (P < 0.001) for systolic BP and

r = 0.76 (P < 0.001) for diastolic BP, with correlations

between CBP and ABPM being systolic r = 0.59

(P < 0.001), diastolic r = 0.47 (P < 0.001).

However, Bland and Altman's method [10] makes a

more appropriate statistical assessment. The data were

examined for the bias, comparing the individual results for

each subject as obtained by the three methods. This

assumes the ABPM to be the true reading and analyses

the spread of individual differences between this result,

CBP and HBPM result (Figs 1 and 2). The mean differences

(bias) between CBP and ABPM were +10.9/+3.8, between

HBPM and ABPM +8.2/+3.7. The 95% Bland and Altman

range for the difference between CBP and ABPM in systolic

BP was ±18.6 to 40.4. Thus, 95% of the time, differences

between CBP and ABPM for systolic BP will lie between

18.6 mmHg below, and 40.4 mmHg above the true BP as

de®ned by ABPM. This range would not be clinically

acceptable. The 95% Bland and Altman ranges for the

difference between HBPM and ABPM are better

Difference from ABPM

Mean

(95% CI)

Bland and Altman precision and 95% range for differences

Upper estimated

value (95%CI)

Lower estimated

value (95% CI)

CBP±ABPM

Sys +10.9 ±18.6 (±25.5, ±11.7) +40.4 (33.5, 47.3)

(6.9, 14.8)

Dias +3.8 ±20.8 (±24.8, ±16.8) +13.1 (9.2, 17.1)

(±6.1, ±1.6)

HBPM±ABPM

Sys +8.2 ±7.8 (±11.6, ±4.0) +24.1 (20.4, 27.9)

(6.0, 10.3)

Dias +3.7 ±16.1 (±19.0, ±13.2) +8.7 (5.8, 11.6)

(±5.4, ±2.0)

Table 2 Differences for clinical blood

pressure measurement (CBP) and home blood

pressure monitoring (HBPM) when compared

with ambulatory blood pressure monitoring

(ABPM) (all values in mmHg)

Table 1 Subject characteristics; details of anti-hypertensive and

diabetic treatments; details of proteinuria (all values as number

(percentage, unless otherwise stated))

Sex: Male 30 (54%)

Female 25 (46%)

Age (years)Ðmean (min±max) 58 (32±72)

Anti-hypertensive treatment:

None 25 (45%)

Single anti-hypertensive agent 17 (31%)

Combination of two agents 10 (18%)

Combination of three agents 3 (5%)

Type 2 diabetes treatment

Diet alone 6 (11%)

Oral hypoglycaemics alone 19 (35%)

Insulin alone 27 (49%)

Insulin + oral hypoglycaemics 3 (5%)

Proteinuria

Dipstick positive 13 (24%)

Original article 433


(±7.8, +24.1). The precision with which the lower and

upper limits of the 95% range are estimated are shown in

Table 2.

There was no difference in HBPM results between days 1

and 4.

Hypertension

Clinically it is important to distinguish patients who will

bene®t from intervention. We de®ned hypertension in our

patient group as per the 1999 British Hypertension

Guidelines [8]. HBPM monitoring showed 100% sensitiv-

ity and 79% speci®city in contrast to CBP (85% sensitivity

and 46% speci®city, i.e. by this measurement 54% of our

subjects had `white-coat hypertension' and would have

been unnecessarily treated). This results in a 90% positive

predictive value for HBPM, compared with 58% for CBP

measurement.

Dipper status

CBP is unable to assess dipper status. HBPM showed

good sensitivity for detecting non-dipper status by

comparing HBPM value at 13.00 and 23.00, but had

very poor speci®city: the sensitivity was 95%, speci®-

city 0% and positive predictive value 78%. The

daytime:night-time ratios for ABPM and HBPM were

signi®cantly different (ABPM mean (SD): systolic 1.03

(0.06), diastolic 1.07 (0.07); HBPM: systolic 1.00

(0.06), diastolic 1.02 (0.06), P < 0.001).

Figure 1 Comparisons between measurements of systolic BP. (a) Clinical blood pressure measurement (CBP) and ambulatory blood pressure

monitoring (ABPM). (b) Home blood pressure monitoring (HBPM) and ABPM.



Discussion

Knowledge of the BP of patients with Type 2 DM is central

to their care; however, obtaining reliable estimates of BP

can be a frustrating experience for clinicians and patients

alike. White-coat hypertension is so prevalent, and the

goals of control post-UKPDS [1] so tight, that BP measured

at a hospital clinic visit rarely informs clinical management

but condemns the patient to multiple visits to the clinic or

the general practitioner. If the clinic monitors the ef®cacy

of treatment, the problem is compounded.

The aim of our study was to evaluate HBPM using a

standard cheap BP monitor available on the high street to

see if this is a practical alternative strategy. This could then

be incorporated into the annual review process.

Consecutive patients were recruited from the routine

diabetes clinic of a large city, so that the results of this

study could apply to most UK diabetic populations. HBPM

was explained to them by the research nurse, adding

approximately 5±10 min to the time of the clinic visit, a

period easily comparable to the time taken to obtain a

standard BP measurement. Our patients generally found

little dif®culty in using the BP monitor (only one out of 58

was unable to operate the monitor). If this became routine

management, it is likely that the time required educating

the patients would be reduced, and the accuracy of the

results may improve.

Twenty-four-hour ABPM was used as a gold standard,

and for the purposes of the study mean BP by this method

was considered the true BP. HBPM was conducted on the

next 4 days. Biological variation plus reproducibility of the

method would result in differences between mean BP in the

same patient even if ABPM were repeated on a consecutive

day. Previous studies of the reproducibility of measure-

ments by the Spacelabs 90207 system have shown standard

deviations of the differences (SDD), a measure of reprodu-

Figure 2 Comparisons between measurements of diastolic BP. (a) Clinical blood pressure measurement (CBP) and ambulatory blood pressure

monitoring (ABPM). (b) Home blood pressure monitoring (HBPM) and ABPM.



cibility, of 6±8 mmHg for systolic BP and 4±6 mmHg for

diastolic BP [11,12]. The bias (i.e. mean of individual

differences) of HBPM compared with ABPM is very similar

to these ®gures, which suggests that the reproducibility

differences in ABPM could account for some of the

difference between HBPM and ABPM. However, the bias

of CBP compared with HBPM is well outside the

reproducibility measurements for ABPMÐthe difference

between CBP and ABPM could not be accounted for by

reproducibility differences in ABPM.

In our study the mean difference between HBPM and

ABPM was +8.2/+3.7 mmHgÐthis is more accurate than

CBP, which when compared with ABPM shows a mean

difference of +10.9/+3.8 mmHg. The bias for HBPM fell

within our predetermined limits of 6 10/5 mmHg, whilst

that of CBP did not. However, whilst the 95% Bland and

Altman range for HBPM with ABPM does not lie within

this prede®ned range, this could be partly explained by

day-to-day physiological BP variation and the reproduci-

bility of ABPM. The 95% Bland and Altman range with

ABPM is much tighter for HBPM monitoring compared

with CBP measurement, and the con®dence intervals

around these limits do not overlap for systolic BP

measurement, suggesting that there is a difference between

HBPM and CBP measurement, with HBPM being super-

ior. We also tested the modulus of the differences between

CBP and ABPM, and HBPM and ABPM, and the results of

these tests suggest that HBPM was signi®cantly closer to

ABPM than CBP, making HBPM a superior measuring

tool for BP.

Despite HBPM failing to satisfy our prede®ned limits of

agreement, its use would dramatically improve clinical

practice. HBPM missed no patients with hypertension

(100% sensitivity), compared with 15% missed by CBP

measurement (85% sensitivity). HBPM falsely identi®ed

21% of normotensive patients as hypertensive (79%

speci®city)Ðhowever, 54% of normotensive patients

were identi®ed as hypertensive by CBP measurement

(46% speci®city). Most patients would therefore be spared

regular clinic review if HBPM was adopted, a bene®t for

both the patient and the overworked clinic.

During this study we found a high percentage of patients

with `white-coat hypertension' (54% on CBP measure-

ment). This is high compared with non-diabetic popula-

tions, possibly due to the effect on medial sclerosis in

diabetic subjects, resulting in reduced arterial compliance,

thus amplifying the white-coat effect. Therapeutic deci-

sions in UKPDS and other BP-lowering trials were made

on the basis of clinic BP measurements, and as always

there is concern that we should not try to exclude patients

who would have been treated in such trials. The protocols

for BP measurement in these trials were strictly monitored

and adhered to, and we regret that the conditions of these

trials are not easily reproducible in the clinic setting. We

would suggest that the results of HBPM provide a better

assessment of true BP in aiding management of hyperten-

sion than CBP in a standard clinic.

In asking the patient to monitor their BP until they went

to bed, we hoped that we could identify those patients who

lose nocturnal BP variation on ABPM (`non-dippers').

ABPM requires BP measurement throughout the night,

which is inconvenient for the patient. However, whilst

HBPM was highly sensitive, it was not speci®c. Thus we

cannot recommend that HBPM be used for this purpose.

In conclusion, this study has shown that home BP

monitoring using a cheap semiautomatic cuff-oscillo-

metric BP monitoring device is superior to clinic BP

measurement, and allows far superior detection of hyper-

tension. Clinic BP measurement revealed `white-coat

hypertension' in over half of the sampled patients, which

could lead to widespread over-treatment of hypertension.

It would seem a reasonable strategy to incorporate Home

Blood Pressure monitoring in the preinvestigation of

patients prior to an annual review along with an estimate

of glycaemic control.

Acknowledgements

We would like to thank most sincerely the Non-Invasive

Cardiology Department at the Royal South Hants

Hospital, Southampton, for their invaluable help. We

would also like to thank Sr. Vivien Foster for her help.

References

1 United Kingdom Prospective Diabetes Study Group. Tight blood

pressure control and risk of microvascular and macrovascular

complications in type 2 diabetes: UKPDS 38. Br Med J 1998; 317:

703±713.

2 Stanton A, O'Brien E. Noninvasive 24-hour ambulatory blood

pressure monitoring: current status. Postgrad Med J 1993; 69:

255±267.

3 Kjeldsen SE, Moan A, Petrin J, Weber AB, Zwei¯er AJ, Julius S.

Evaluation of self-measured home vs. clinic intra-arterial blood

pressure. Blood-Press 1993; 2: 28±34.

4 Stergiou GS, Malakos JS, Voutsa AV, Achimastos AD,

Mountokalakis TD. Home monitoring of blood pressure: limited

value in general practice. J-Hum-Hypertens 1996; 10: 219±223.

5 Stergiou GS, Voutsa AV, Achimastos AD, Mountokalakis TD.

Home self-monitoring of blood pressure: is fully automated

oscillometric technique as good as conventional stethoscopic

technique? Am-J-Hypertens 1997; 10: 428±433.

6 Brueren MM, de Schouten HJA, Leeuw PW, van Montfrans GA, van

Ree JW. A series of self-measurements by the patient is a reliable

alternative to ambulatory blood pressure measurement. Br J Gen

Prac 1998; 48: 1585±1589.

7 O'Brien E, Mee F, Atkins N, O'Malley K. Accuracy of the Spacelabs

90207 determined by the British Hypertension Society Protocol. J

Hypertens 1991; 9: S25±S31.

8 Ramsay LE, Williams B, Johnston GD, MacGregor GA, Poston L,

Potter JF et al. British Hypertension Society guidelines for hyperten-

sion management: summary. Br Med J 1999; 319: 630±635.

9 Williams B. The pathogenesis of hypertension in diabetes mellitus.



In: Diabetes and HypertensionÐa Fatal Attraction Explained.

Maidstone: Publishing Initiatives Books, 1997: 26±27.

10 Bland JM, Altman DG. Statistical methods for assessing agreement

between two methods of clinical measurement. Lancet 1986; 327:

307±311.

11 Fotherby MD, Potter JF. Reproducibility of ambulatory and clinic

blood pressure measurements in elderly hypertensive subjects. J

Hypertens 1993; 11: 573±579.

12 Engfeldt P, Danielsson B, Nyman K, Aberg K, Aberg H. 24-hour

ambulatory blood pressure monitoring in elderly normotensive

individuals and its reproducibility after one year. J Hum Hypertens

1994; 8: 545±550.



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