NPC Archive Item: Concerns raised over the accuracy of blood pressure measurement devices.

27 September 2011

An observational study¹ in 38 primary care practices in Oxfordshire has found that overall, 1 in 7 sphygmomanometers (sphygs) failed to meet the British Hypertension Society standard (all readings within 3mmHg of the reference standard). The performance of mercury and digital devices was similar and significantly better than aneroid devices. Aneroid devices donated by pharmaceutical companies were particularly inaccurate: fewer than half met the standard.

Level of evidence:
Level 3 (other evidence) according to the SORT criteria.

Action
Healthcare providers should follow NICE guidance on hypertension which states that devices for measuring blood pressure should be properly validated, maintained and regularly recalibrated according to manufacturers’ instructions. This study¹ highlights the importance of this, particularly for aneroid devices and especially those provided by pharmaceutical companies. These should be replaced with high quality devices as recommended by the British Hypertension Society (BHS) who provide a list of validated devices.

Healthcare professionals taking blood pressure measurements should also ensure that they follow NICE guidance on measuring blood pressure and ensure they have adequate initial training and periodically review their performance.

What is the background to this?
Hypertension is one of the most important preventable causes of premature morbidity and mortality in the UK. It is a major risk factor for ischaemic and haemorrhagic stroke, myocardial infarction, heart failure, chronic kidney disease, cognitive decline and premature death. Diagnosis and control of hypertension relies upon accurate blood pressure measurement and depends upon both the device and technique used. Inaccurate blood pressure measurement could lead to misdiagnosis and either unnecessary or insufficient treatment. This study¹ assessed the type and accuracy of equipment used for monitoring blood pressure in 38 primary care practices in Oxfordshire.

What does this study claim?
A total of 604 devices were identified: 323 digital (53%), 192 aneroid (32%), 79 mercury (13%), and 10 hybrid (2%) devices. Of these, 584 (97%) could be fully tested.

Overall, 14% of devices failed to meet the BHS standard that requires all devices to read within 3mmHg of the true value; 13% had one or more errors of 4–9mmHg; and less than 1% had one or more errors of more than 10mmHg. The performance of mercury and digital sphygs was not significantly different: 5% of mercury devices and 12% of digital devices failed to meet the standard. Significantly more aneroid devices failed to meet the standard (22%) compared with mercury and digital devices (p=0.001). Aneroid devices supplied by pharmaceutical companies performed particularly poorly: 57% of these did not meet the standard, compared with 16% of aneroid devices where the device manufacturers’ name was actually stated on the device.

Where devices failed to meet the standard on testing, there was some evidence that errors were present across a range of blood pressures, leading to consistent over- or under-diagnosis or treatment in clinical practice. Conversely, instruments providing accurate readings at one pressure tended to be accurate across the range.

No significant differences were seen between devices from commonly encountered manufacturers. There was little evidence that previous calibration made any difference to current accuracy, although just over half had been checked within the last year.

How does this relate to other studies?
Previous studies of sphygs in primary and secondary care have suggested that aneroid devices have inferior accuracy but they did not comprehensively address digital devices.

So what?
A significantly higher failure rate was detected for aneroid sphygs, particularly when the analysis was restricted to aneroid devices apparently received as gifts from pharmaceutical representatives. The authors speculate that such devices are likely to be low-cost, possibly inferior products because pharmaceutical industry regulations only allow donation and acceptance of inexpensive promotional items.

Although previous calibration did not appear to make any difference to current accuracy, this might be because the ‘never tested’ group included not only older devices (including the four most inaccurate devices) but also newer devices within the manufacturer’s warranty. Also, within the ‘previously tested’ group, there were some devices that had previously failed, which, worryingly, had been retained.

This observational study has some limitations. Testing was limited to one geographical area that may not be representative of the country as a whole. Seven of the practices had previously participated in a pilot for this study and so were likely to perform better. In addition, practices that declined to participate in the study may be less interested in the topic and, therefore, less likely to maintain their equipment. Worryingly, these potential biases could have led to overestimation of the accuracy of devices in practice.

The results of the study suggest that poor quality aneroid devices, such as those provided by the pharmaceutical industry, should be replaced with high-quality validated devices. BHS provides a list of validated devices. It also offers advice on measuring blood pressure, as does NICE in its hypertension guideline. The MHRA produced a bulletin on blood pressure measurement devices in July 2006, most of which is still valid and relevant today.

1. A’Court C, et al. Type and accuracy of sphygmomanometers in primary care: a cross-sectional observational study. Br J Gen Pract 2011;61:e598–603 (subscription needed to view paper)

Study details

Design: Cross-sectional observational study in primary care practices in Oxfordshire in 2009. Of 83 practices invited to take part, 38 (46%) agreed.

Intervention and comparison: Sphygs were evaluated between 50 and 250mmHg using two types of pressure testing device which had been confirmed equivalent.

A ‘traffic light’ classification of error readings was devised, in which devices were assigned a ‘green’ rating if all results fell within 0–3mmHg of the reference blood pressure (the BHS standard), ‘amber’ if one or more readings deviated by 4–9mmHg, and ‘red’ if any of the results differed by 10mmHg or more.

Outcomes and results: 604 devices were identified: 323 digital (53%), 192 aneroid (32%), 79 mercury (13%), and 10 hybrid (2%) devices. Of these, 584 (97%) could be fully tested (i.e. were usable and compatible with either testing device).

Overall, 86% of devices were within 3mmHg of the BHS standard across the BP range; 13% had one or more error of 4–9mmHg, and less than 1% had one or more error of 10mmHg or more.

Table: Accuracy of types of BP measuring devices

Compared to mercury, digital devices were no more likely to fail the BHS standard

(relative risk [RR] = 2.19, 95% confidence interval [CI] = 0.80 to 5.97), but aneroid

models were four times more likely to fail (RR = 4.02, 95%, CI = 1.49 to 10.8).

No significant difference was found in performance between manufacturers within each device type, for either aneroid (p= 0.96) or digital (p=0.7) devices.

Overall, no significant difference was found between testing date and accuracy of device: a ‘green’ rating was achieved in 87% of devices tested in the last 6 months, 89% of those tested within 6 to 12 months, 87% of those tested over 12 months before, and 80% of those never tested (p=0.1).

Sponsorship: National Institute of Health Research

Further information can be found on NHS Evidence and in the cardiovascular disease – hypertension e-learning section of the NPC website.

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