Ambulatory blood pressure monitoring (ABPM) is. Accuracy of Ambulatory Blood Pressure Monitors in Routine Clinical Practice.

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1 AJH 26; 19:81 89 BP Measurement Accuracy of Ambulatory Blood Pressure Monitors in Routine Clinical Practice Tony C. Y. Pang and Mark A. Brown Background: To determine the extent of discrepancies between ambulatory blood pressure measurement (ABPM) devices and mercury sphygmomanometry in pre-use testing in routine clinical practice and whether such discrepancies are associated with patient characteristics and subsequent 24-h ABPM readings. Methods: A retrospective analysis of a database of 683 prospectively collected records was performed. The study population comprised of patients referred for ABP monitoring at a Sydney teaching hospital. Patients had same-arm sequential measurements with a mercury sphygmomanometer and Spacelabs 927 or 9217 device before 24-h ABPM. The discrepancy between the test devices and the standard mercury sphygmomanometer were then described by two measures, calculated for both systolic and diastolic pressures for each patient: 1) difference of means ( BP average), and 2) the mean of differences between device measurement and the range of mercury sphygmomanometer readings, analogous to the British Hypertension Society protocol ( BP sequential). The main outcome measures were BP average and Ambulatory blood pressure monitoring (ABPM) is used increasingly in routine clinical practice. Ambulatory BP is regarded as superior to conventional BP measurement because it measures BP repeatedly outside of a clinical environment giving a better reflection of the patient s true BP and eliminates the white coat effect. 1,2 In addition, because of the larger volume of data obtained over time, prognosis is not limited to just a single BP value but also to patterns of the diurnal cycle, nondippers having a higher likelihood of cardiovascular and cerebrovascular complications. 3,4 Prospective studies have shown that ambulatory BP is a better predictor of cardiovascular morbidity and mortality than conventional BP measurements 5 7 and that less antihypertensive medications are prescribed if the diagnosis of hypertension is based on ABPM. 8 The ABPM also helps select those BP sequential and their relationship to patient characteristics. Results: The median BP average was 1/2 mm Hg (ABPM device underestimation) and the median BP sequential was 3/2 mm Hg. Age, gender, arm circumference, body mass index, and degree of hypertension influenced the accuracy of ABPM readings on multivariate analysis. Device accuracy was slightly weaker in patients with higher mercury or ABPM-derived systolic BP. Conclusions: These ABPM devices are accurate enough for routine clinical use in a variety of patients. Factors such as age, weight, gender, and severity of hypertension are statistically associated with greater device error but the differences are small enough to be unlikely to affect clinical practice. Am J Hypertens 26;19: American Journal of Hypertension, Ltd. Key Words: Hypertension, ambulatory blood pressure monitoring, accuracy, Spacelabs, age, gender, arm circumference, body mass index. subjects with true refractory hypertension for further investigations. 9 A variety of ABPM machines are available, and it is recommended that all be validated according to established protocols such as that of the British Hypertension Society (BHS) 1 or the Association for the Advancement of Medical Instrumentation (AAMI). 11 Although these protocols validate the overall accuracy and reliability of these machines, it is not known whether this accuracy translates to individual patients in everyday clinical practice. The aim of this study was to determine the preuse accuracy of two commonly used ABPM devices compared with mercury sphygmomanometry in routine clinical practice and to investigate patient characteristics that may affect any discrepancy between these oscillometric ABPM devices and mercury sphygmomanometry. Received October 2, 25. First decision December 28, 25. Accepted December 29, 25. From the Departments of Renal Medicine and Medicine, St. George Hospital & University of NSW, Kogarah, Sydney, Australia. This study was supported by the Department of Renal Medicine, St. George Hospital. No funding was supplied by Spacelabs. Address correspondence and reprint requests to Prof. Mark Brown, Department of Renal Medicine, St. George Hospital, Kogarah NSW 2217 Australia; mbrown@unsw.edu.au 26 by the American Journal of Hypertension, Ltd. Published by Elsevier Inc /6/$32. doi:1.116/j.amjhyper.25..

2 82 FACTORS ASSOCIATED WITH ABPM ACCURACY AJH August 26 VOL. 19, NO. 8 Methods Setting and Participants This study is a retrospective analysis of a database of information collected at the time of each ABPM at St. George Hospital, a teaching hospital in Sydney. This database contains 2833 records of ABPMs performed since Patients are referred either by a hospital physician or general practitioner, the major indications for referral being diagnosis of ambulatory hypertension or evaluation of BP control. The BP monitoring unit in the hospital is operated by two registered nurses highly trained in the use of mercury sphygmomanometers and ABPM devices. Since 21 we have routinely tested the discrepancy between the ABPM devices and the standard mercury sphygmomanometer (device sphygmomanometer discrepancy) before their use in every patient. Informed consent regarding the collection of data was obtained from all patients. Records from these 693 subjects (from 21 to 24) form the basis of this analysis. Equipment We used 1 ABPM devices (nine Spacelabs 927 devices and one 9217; Spacelabs Medical, Dee Why, Sydney, Australia) for the 24-h readings. Both of these models have been validated according the BHS protocol.,13 For comparison with the ABPM devices, we used a regularly calibrated and maintained mercury sphygmomanometer. The ABPM devices are recalibrated approximately every 6 months. Large cuffs were used for both mercury sphygmomanometry and ABPM if the midupper arm circumference was 33 cm. Although small cuffs for small arms may have improved accuracy, we did not record how often this was needed, but this is an unusual occurrence in our practice. BP Measurements Before the start of 24-h BP monitoring, BP measurements were made alternately using the ABPM device and the mercury sphygmomanometer for a total of seven readings (4 mercury sphygmomanometer readings interspersed with 3 ABPM readings) on the same (nondominant) arm. All measurements were made by the BP unit registered nurses in a quiet office environment. Two measures of accuracy were calculated: 1) the difference of the means of BP measured by the mercury sphygmomanometer and the ABPM device ( BP average) and 2) the BP difference according to whether the ABPM device recorded a BP within the range of mercury sphygmomanometer BP measurements immediately before and after that ABPM reading, analogous to the method recommended by the BHS. 1 If the ABPM readings were within this range, then a difference of zero was recorded. For readings outside this range, the difference was the mm Hg ABPM measurement outside the closest mercury reading. No attempt was made to distinguish between values above or below the range, that is, all differences were absolute values. The three differences obtained by this method were then averaged to give the mean difference ( BP sequential). During the subsequent 24 h, ABPM measurements were taken every half hour with patients blinded to these recordings. Awake, sleep, and 24-h average BP was obtained along with BP loads. Patients reported their sleep times in a diary, which were used to calculate separate awake and sleep BP averages. Statistical Analysis Recorded data included BP average and BP sequential along with age, gender, arm circumference, device used, body mass index (BMI), 24-h mean systolic and diastolic BP, awake mean systolic and diastolic BP, sleep mean systolic and diastolic BP, and whether or not the patient was taking antihypertensive medications. We recorded BMI as an indicator of obesity as it has been recently demonstrated that the use of body fat mass has no advantage over BMI in the prediction of obesity-related metabolic risk. 14 However, we did not discern the fat or muscle components of BMI in our cohort. Blood pressure measurements were reported as means and standard deviations (SD), whereas BP average and BP sequential were reported as medians and interquartile ranges (IQR) as these were not normally distributed. Analyses were made with patients categorized into groups according to age ( 3, 3 to 49, 5 to 69, 7 years); arm circumference ( 33 cm, 33 cm); BMI (normal 25, overweight 25 to 29.9, obese 3 kg/m 2 ); and 24-h mean BP ( 13/8, 13 to 159/8 to 99, 16/1 mm Hg). Device sphygmomanometer frequency were described by a Bland-Altman plot and analyzed by linear regression analysis. Univariate analysis of baseline characteristics and BP average and BP sequential were tested with unpaired t-test or one-way ANOVA as appropriate. Multivariate analysis of the relationship between patient characteristics and BP average or BP sequential was performed with multiple regression analysis. Multivariate analysis of the associations between sphygmomanometer-measured office BP, 24-h BP, awake BP, and sleep BP with BP average and BP sequential were performed individually to account for patient characteristics thereby avoiding problems with collinearity. Analyses were calculated and plotted with GraphPad Prism 4.1 (univariate analyses) and GraphPad InStat 3.6 for Windows (multivariate analyses) (GraphPad Software, San Diego, CA). The study was approved by the South Eastern Sydney and Illawarra Area Health Service Human Research Ethics committee (Southern Section) as a quality assurance study not requiring independent ethical review.

3 AJH August 26 VOL. 19, NO. 8 FACTORS ASSOCIATED WITH ABPM ACCURACY 83 Table 1. Blood pressures measured before and during 24-h ambulatory blood pressure monitoring (ABPM) Systolic BP Diastolic BP Preuse testing Mercury 143 (18) 83 () ABPM 142 (17) 81 () In use 24-h 131 (14) 76 (1) Awake 134 (14) 79 (11) Sleep 1 (16) 68 (1) Data are mean (SD). Results Study Population Of the ABPM records of 693 subjects, 1 were excluded from analysis due to missing data or impossible values, leaving 683 for analysis. The 683 subjects included 399 women and 284 men. Mean (SD) age was 55 (17) years; A Frequency (n) Frequency (n) Systolic BP-average (mmhg) BMI 27.4 (5.4) kg/m 2 ; arm circumference 3 (SD 3.7) cm. Two hundred twenty-five subjects were referred for the diagnosis of hypertension, 439 for evaluation of BP control, and 19 for other reasons. At the time of ABPM 448 subjects were taking one or more antihypertensive medications. The mean BPs before and during use of ABPM are shown in Table 1. BP Average and BP Sequential The mean systolic and diastolic BP average were 1.4 mm Hg (95% confidence interval [CI] ) and 2.1 mm Hg (95% CI ), respectively. The mean systolic and diastolic BP sequential were 3.2 mm Hg (95% CI ) and 3.2 mm Hg (95% CI ), respectively (Fig. 1). The Bland-Altman plot of preuse systolic and diastolic measurements are shown in Fig. 2. Linear regression analysis demonstrates a statistically significant non-zero slope for both systolic and diastolic preuse measurements. The slopes (95% CI) are.64 (.4.89) (P.1) and.37 (.7.45) (P.5), respectively. B Frequency (n) Frequency (n) Systolic BP-sequential (mmhg) Diastolic BP-average (mmhg) Diastolic BP-sequential (mmhg) FIG. 1. (A) Distribution of systolic and diastolic BP average. (B) Distribution of systolic and diastolic BP sequential. See text for definitions.

4 84 FACTORS ASSOCIATED WITH ABPM ACCURACY AJH August 26 VOL. 19, NO. 8 A B Systolic BP-average (mmhg) Diastolic BP-average (mmhg) Mean systolic BP (mmhg) Mean diastolic BP (mmhg) FIG. 2. Bland-Altman plot describing the relationship between the device sphygmomanometer discrepancy and blood pressure (BP) (mean of mercury and ambulatory BP device measured BPs). (A) Systolic: mean (SD) of bias 1.4 (5.6) mm Hg, 95% limit of agreement is from 9.8 to.7 mm Hg. (B) Diastolic: mean (SD) of bias 2.1 (5.1) mm Hg, 95% limit of agreement is from 7.9 to.1 mm Hg. Note: Linear regression line has been plotted as the solid line with 95% confidence intervals (dotted curves); the additional Y-axis tick represents the mean bias of the sample. Relationship Between Patient Characteristics and Device Sphygmomanometer Discrepancy In univariate analyses (Table 2), systolic BP average was greater in women and increased with older age and smaller arm circumference. Diastolic BP average, however, tended to be higher with larger arm circumference and higher BMI. The device discrepancy of systolic BP sequential was higher if patients were taking antihypertensives, whereas diastolic BP sequential was greater in women, those with large arm circumference and high BMI. In all of these groups the absolute difference, although statistically significant, ranged from only 1 to 4mmHg. Multiple regression analysis confirmed statistically significant relationships between systolic BP average with age and arm circumference but not gender. Diastolic BP average varied significantly with gender and arm circumference but not with BMI. Systolic BP sequential was not associated with any patient characteristics, whereas diastolic BP sequential yielded the same associates as for univariate analysis (ie, gender and arm circumference as well as BMI). The ABPM device used did not significantly affect systolic or diastolic BP sequential or diastolic BP average. Systolic BP average was also largely unaffected by the specific ABPM device used; however, device numbers 3 and 4 did demonstrate statistically significant differences compared to a number of other devices. The means (SD) of BP sequential and BP average for each of the 1 machines used are displayed in Table 3. BP and Device Sphygmomanometer Discrepancy Multivariate analysis showed that systolic BP average and systolic BP sequential was slightly greater with higher mercury measured systolic pressure. Diastolic BP average was slightly greater (1 to 3 mm Hg) in those with lower 24-h mean diastolic BPs. The diastolic BP sequential was similar across all mercury and ABPM BP ranges (Table 4). Device Accuracy as per BHS Criteria Table 5 shows that these devices collectively received a grade A rating for preuse accuracy as per BHS criteria. Discussion This study shows that BP measurements made with the ABPM device tended to underestimate BP measured with mercury sphygmomanometry but only by 1/2 mm Hg.The ABPM readings also deviated significantly from the range of values measured by the mercury sphygmomanometer in each patient (the BP sequential) by a median of 3/2 mm Hg. Although even small differences in BP are clinically relevant for a population, such small differences in BPs are unlikely to be clinically relevant for an individual in terms of routine clinical practice decisions. Although both BP average and BP sequential are measures of device sphygmomanometer discrepancy, their exact meanings differ somewhat. The directional nature of a simple difference of means ( BP average) is probably more useful in giving us information regarding bias, for instance, if the ABP device were consistently underestimating or overestimating BPs. On the other hand inaccuracies that are not consistent in direction may be cancelled out in this measure, giving a false impression of accuracy. Therefore the BP sequential is probably more useful in detecting overall inaccuracy,

5 AJH August 26 VOL. 19, NO. 8 FACTORS ASSOCIATED WITH ABPM ACCURACY 85 Table 2. Univariate analyses of differences between mercury blood pressure and ABPM device according to patient characteristics n Systolic Diastolic Systolic Diastolic Gender Male ( 3 4) 2 (.5 4) 3 (1 4) 2 (1 3) Female ( 2 5) 2 ( 1 6) 3 (1 4) 3 (1 5) P Age (y) ( 3 2) 2 ( 1 5) 2 (1 3) 3 (1 4) ( 3 3) 2 ( 1 5) 2 (1 4) 2 (1 4) ( 2 5.5) 2 ( 5) 3 (1 5) 3 (1 4) ( 1 7) 1 ( 1 4) 3 (1 5) 2 (1 4) P Arm circumference ( 2 4) 2 ( 1 4) 3 (1 4) 2 (1 4) ( 3 4) 4 ( 7) 2 (1 4) 3 (2 6) P BMI (kg/m 2 ) ( 2 5) 1 ( 1 4) 3 (1 4.5) 2 (1 4) ( 2 5) 2 ( 1 4) 3 (1 4) 2 (1 4) ( 3 4) 3 ( 7) 2 (1 5) 3 (1 5) P Referral reason 1. Diagnosis of HT ( 2 4) 2 ( 1 5) 2 (1 4) 2 (1 4) 2. Evaluation of control ( 2 6) 2 ( 1 5) 3 (1 5) 2 (1 4) 3. Other 19 ( 3 5) ( 2 3) 2 (1 4) 2 (1 4) P Antihypertensive No ( 2 4) 2 ( 1 5) 2 (1 4) 2 (1 4) Yes ( 2 6) 2 ( 1 5) 3 (1 5) 2 (1 4) P Data are median (IQR). systematic or otherwise. As expected, there was a significant correlation between these two measurements of error. Other investigators have measured directional differences between mercury and a variety of ABPM measured BP in smaller numbers of subjects in studies specifically designed to test such accuracy. Santucci et al 15 studied 45 patients and found only small BP differences for the Del Mar Avionics Pressurometer IV and the Spacelabs 922. Baumgart and Kamp found an average difference of 5 mm Hg for systolic BP using the Spacelabs 9217 in 85 subjects, whereas Palatini et al 16 assessed the TM-243 device using a method similar to BP sequential and found good accuracy in 98 subjects. We have also demonstrated clinical accuracy for the Spacelabs 927 and 9217 in everyday practice in almost 7 subjects. It is important to acknowledge that different results are obtained when intra-arterial BP monitoring is the gold standard. Groppell et al 17 found that the Spacelabs 922 and 927 devices accurately reflected systolic but overestimated diastolic BP. We found the same phenomenon in pregnant women. 18 BP average Factors That Affect Accuracy of ABPM Devices BP sequential A number of previous studies have investigated factors affecting the accuracy of ABPM and other oscillometric devices, but differences in methodology and device models have made results difficult to compare. A major problem of methodology has been whether sequential or simultaneous same-arm measurements were used in device comparison. Although it is known that simultaneous measurements produce results with less device-observer discrepancy, 19 it is not known whether this difference affects all measurements uniformly or is patient-characteristic or devicedependent. Furthermore, interpretation of studies also depends on whether device mercury discrepancies were measured as difference of means or as a nondirectional measure similar to that we have used. Our study shows (particularly in Table 5) that these probably represent two different aspects of measuring accuracy. A review of the literature suggests some patient characteristics that affect agreement between oscillometric and mercury devices. One study that tested Spacelabs 927

6 86 FACTORS ASSOCIATED WITH ABPM ACCURACY AJH August 26 VOL. 19, NO. 8 Table 3. Device-Sphygmomanometer discrepancies for each of the ten devices. All are Spacelabs 927 devices except device 9 (Spacelabs 9217). Data are mean (SD). Device number n BP average Systolic 8. (7.) 1. (5.2).2 (5.) 3.2 (6.) 4.2 (5.3).9 (6.3).1 (5.) 1.6 (5.7).5 (5.3).8 (6.6) Diastolic 1.5 (3.6) 2. (4.1) 1.8 (5.5) 2.7 (5.2) 2.6 (4.9) 2. (5.4) 2.5 (4.6) 2.1 (4.3) 1.8 (6.3) 1.9 (2.5) BP sequential Systolic 5. (5.5) 2.8 (2.2) 2.7 (2.2) 3.6 (3.4) 3.4 (2.6) 3.4 (3.1) 2.9 (2.4) 3.3 (3.2) 3. (2.2) 3.9 (3.4) Diastolic 3.5 (2.3) 2.7 (1.6) 3.4 (3.3) 3.8 (3.3) 3.2 (3.) 3.2 (3.1) 2.7 (2.6) 3.1 (2.2) 3.6 (4.).8 (.5) devices 2 in an elderly population (6 to 9 years) with sequential arm readings suggested that this device was not as accurate in the elderly for measuring systolic BP, whereas diastolic BP measurements were satisfactory (according to BHS criteria). We found this same trend, with increasing age associated with greater device discrepancy as measured by systolic BP average. However, we found no significant difference in systolic or diastolic accuracy with age when using the BP sequential, suggesting that age causes underestimation by this device rather than increasing its inaccuracy. As such, we doubt that this is a clinically meaningful finding. Validation studies with some other ABPM device models have not demonstrated this age-related effect, 16,21 although both these studies were performed using simultaneous opposite arm readings. Brinton et al 22 examined the effect of age in 154 subjects aged between 11 and 85 years and found that the older age group was associated with less disagreement between the two methods. We also found gender-related difference whereby diastolic (but not systolic) error ( BP sequential) was significantly greater in women. It is not clear why this should be the case but as for age, the magnitude of this difference was such that it is unlikely to affect clinical practice. Large arm circumference has been associated with increased device observer discrepancy. 16,19,21 Our data show that patients with arm circumferences greater than 33 cm had a statistically significant but clinically small overestimation of systolic BP and underestimation of diastolic BP by the ABPM devices. Similarly, BMI influenced ABPM accuracy, with diastolic BP sequential increasing with BMI, even after accounting for arm circumference. Again, although these were statistically meaningful observations, the magnitude of difference was such that it is unlikely to affect interpretation of ABPM findings in an individual patient. Importantly, we found that use of antihypertensive medications did not affect device sphygmomanometer BP discrepancy, meaning that patients having ABPM and already taking antihypertensives can have reliable results. This is a reassuring finding for trials of drug therapy using ABPM. Association Between ABPM Accuracy and Subsequent BP Measurements We found that higher mercury systolic BPs were associated with increasing but still small magnitude underestimation of systolic BP by the ABPM device, This is consistent with results from other studies, 2,23 including a systematic review of 3 studies between 1993 and Interestingly, there were no statistically significant association between preuse discrepancy (measured as BP sequential) and subsequent 24-h mean BPs. These findings again suggest that the (small) device mercury differences do not translate into meaningful influences on the final ABPM data. We also noted that higher diastolic BPs were

7 AJH August 26 VOL. 19, NO. 8 FACTORS ASSOCIATED WITH ABPM ACCURACY 87 Table 4. Univariate analyses of differences between mercury blood pressure and ABPM according to sphygmomanometer and ABPM measured blood pressures n Systolic Diastolic Systolic Diastolic 24-h mean systolic BP ( 3 4) 2 ( 6) 2 (1 4) 3 (1 4) ( 2 5) 2 ( 1 4) 3 (1 5) 2 (1 4) ( 5 8) 1 (.5 3) 4 (2 6) 2 (1 3.5) P h mean diastolic BP ( 2 4.5) 2 ( 1 6) 3 (1 4) 3 (1 4) ( 2 4) 2 ( 1 4) 3 (1 5) 2 (1 4) ( 2 6) 1 ( 5 2) 4 (2 5) 2 (1 5) P Mercury systolic BP ( 4 2.5) 2 ( 1 5) 3 (1 4) 3 (1 4) ( 2 4.5) 2 ( 1 5) 2 (1 4) 2 (1 4) ( 9.5) 2 ( 4.5) 4 (2 6) 2 (1 3) P Mercury diastolic BP ( 2 5) 1.5 ( 2 5) 2 (1 4) 3 (1 4) ( 3 4) 2 ( 5) 3 (1 4.5) 2 (1 4) ( 7.5) 2 (1 6.5) 3 (1 5) 3 ( ) P Mercury BP 11/8 87 ( 3 3) 2 ( 1 5) 2 (1 4) 3 (1 4) / ( 3 4) 2 ( 1 5) 2 (1 4) 2 (1 4) 16/ ( 9) 2 ( 5) 3 (1.5 5) 2.5 (1 4) P Data are median (IQR). associated with a decreasing underestimation of diastolic BP by the ABPM device. O Brien et al 25 noted that the level of accuracy of some ABPM devices slipped as BP increased but the Spacelabs 927 remained reasonably accurate. Our data, derived from everyday clinical practice, would support that observation. Table 5. Percentage of measurements of BP sequential and BP average 5 mmhg, 1 mm Hg, and 15 mm Hg Grade % Measurements Within <5 mm Hg <1 mm Hg BP average Median, IQR <15 mm Hg BP average SBP A DBP A BP sequential SBP A DBP A SBP systolic blood pressure; DBP diastolic blood pressure; BHS grade A criteria are 6% 5 mm Hg, 85% 1 mm Hg, and 95% 15 mm Hg. BHS Accuracy Using the BHS criteria as a guide (6% of differences 5 mm Hg, 85% 1 mm Hg, and 95% 15 mm Hg) both measures of assessing accuracy yield grade A/A for the devices collectively (Table 5). It is apparent from Table 5 that the BP sequential method implies greater accuracy within 5 mm Hg and clinicians need to decide which method (ie, the difference of averages or the average of differences) is more meaningful. As discussed the former method probably indicates whether a device has a consistent tendency to underestimate or overestimate BP, whereas the latter is probably a greater reflection of accuracy for routine use. Strengths and Limitations of the Current Study BP sequential Median, IQR The three key elements that distinguish our study from earlier studies comparing mercury and ABPM derived BP are: 1. Analyses using two different methods of assessing accuracy, 2. The study of a very large number of patients, almost 7, and

8 88 FACTORS ASSOCIATED WITH ABPM ACCURACY AJH August 26 VOL. 19, NO The fact that these data are derived from patients referred for ABPM as part of everyday clinical practice, reflecting the whole spectrum of patients seen in general and specialty practices. As with most studies, ours has some limitations. The first is that this is a retrospective analysis of data collected from patients referred to our ABPM unit. The participants are therefore a heterogeneous group, meaning that unknown factors such as co-morbidities (eg, diabetes) could bias our data. 26 Also, as with all studies using sequential BP measurements (like those that follow the BHS protocol), systolic BP discrepancy may be overestimated because systolic BP tends to decrease after the first measurement. 24,27 Because BP readings tend to stabilize after the second measurement, 28 the use of seven successive measurements may have improved our estimates of device accuracy somewhat. Another limitation is that although the Spacelabs 927 and 9217 are widely used ABPM devices, there are many different ABPM and automated oscillometric devices on the market, each with their own algorithm to calculate systolic and diastolic BPs 29 and this will limit the generalizability of this study. As with all static phase validation studies, data obtained at rest may not be generalizable to in-use conditions, for instance, changes in posture may affect the accuracy of ABPM measurements. 3 A final potential limitation is that we do not routinely exclude patients with atrial fibrillation from ABPM study despite the potential for inaccuracy. 31 Rather, these patients undergo ABPM if their pretest BP sequential is within 5 mm Hg. We have not systematically recorded data on these subjects but have had to turn away very few patients with atrial fibrillation and high pretest BP sequential over the years of this service. Conclusion This large study of patients referred for ABPM studies in routine clinical practice has shown that the Spacelabs 927 and 9217 ABPM devices deviate significantly from mercury sphygmomanometry on preuse testing, but these deviations are in the order of only a few mm Hg and are unlikely to be clinically significant for individual patients. Gender, age, arm circumference, BMI, and mercury BP also have a statistically significant affect on the agreement between mercury sphygmomanometer and ABPM machine measurements of BP at rest, but again none of these patient characteristics appear to influence measurements enough to matter clinically and the devices achieved grade A for both systolic and diastolic BP over several years of routine clinical use. These findings suggest that these devices are generally accurate in routine clinical use. References 1. Owens P, Atkins N, O Brien E: Diagnosis of white coat hypertension by ambulatory blood pressure monitoring. Hypertension 1999; 34: Staessen JA, O Brien ET, Thijs L, Fagard RH: Modern approaches to blood pressure measurement. Occup Environ Med 2;57: O Brien E, Sheridan J, O Malley K: Dippers and non-dippers. Lancet 1988;2: Mallion JM, Baguet JP, Siche JP, Tremel F, De Gaudemaris R: Clinical value of ambulatory blood pressure monitoring. J Hypertens 1999;17: Pickering TG: Ambulatory blood pressure monitoring. Curr Hypertens Rep 2;2: Verdecchia P: Prognostic value of ambulatory blood pressure: current evidence and clinical implications. Hypertension 2; 35: Staessen JA, Thijs L, Fagard R, O Brien ET, Clement D, de Leeuw PW, Mancia G, Nachev C, Palatini P, Parati G, Tuomilehto J, Webster J: Predicting cardiovascular risk using conventional vs ambulatory blood pressure in older patients with systolic hypertension. Systolic Hypertension in Europe Trial Investigators. 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