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Millions affected by routine blood pressure monitor errors

By Lilian Anekwe

Serious errors in many of the sphygmomanometers routinely used in general practice could call into question the diagnoses of millions of patients with hypertension, a leading group of GP researchers is warning.

Up to a quarter of all sphygmomanometers may be inaccurate by between 3 and 10mmHg and one in seven by more than 10mmHg, their study found.

The research team claimed systematic errors of as little as 5mmHg either way could half or double rates of hypertension diagnosis.

A comparison of 438 mercury, aneroid and digital sphygmomanometers found frequent errors with both aneroid and digital devices.

The results, presented at the south west Society for Academic Primary Care conference last month, suggest the phase-out of mercury sphygmomanometers and shift towards digital devices could be placing quality of diagnosis at risk.

In 2004 the Department of Health convened a committee to investigate inaccuracies in non-mercury sphygmomanometers.

It warned that despite kite-marking and the introduction of British Hypertension Society standards, many machines would be unfit for purpose unless frequently calibrated.

Five years on, the new research suggests GPs are not calibrating their sphygmomanometers often enough to ensure minimum standards of accuracy. Some 58% of the devices in the study had never previously been tested.

The University of Oxford researchers measured the accuracy of devices used in 19 GP practices and five community hospitals in Oxfordshire PCT.

They found that overall 14% of devices were inaccurate by a factor of 10mmHg or more.

The likelihood of this degree of error was three times as high with digital devices, and almost three times as high with aneroid devices, as with mercury ones.

Study leader Dr Christine A'Court, a GP in Carterton, Oxfordshire, said: ‘The poor performance of aneroid and digital monitoring devices assessed is of concern as a systematic error of just plus or minus 5mmHg can double or at least halve the hypertension diagnoses rates respectively.

‘Inconsistency of blood pressure findings from one device, or between different practitioners' devices may delay diagnosis and cause inappropriate prescribing.'

Professor Andrew Shennan, professor of obstetrics at King's College London and chair of the Department of Health committee on blood pressure measurement in clinical practice, told Pulse it was important GPs remembered to calibrate their sphygmomanometers.

He added: ‘There's evidence an inaccuracy of plus or minus 3mmHg could misdiagnose one in three patients, so their conclusion certainly sounds very plausible to me.

‘Some devices are very good, but it's really important if you're going to use an automated device you must make sure it's validated by the BHS and calibrated. The onus is on us as clinicians to check their accuracy.'

Fellow committee member Dr Peter McCartney, a GP in Bristol, said: ‘There's a pressure to avoid using mercury sphygmomanometers because of toxicity. There's a drift towards digital but there should be some organised calibration system for digital machines.'

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