Higher arm reading ‘overestimates true BP’

By Caroline Price, Senior medwireNews Reporter

Using the higher of left and right arm single blood pressure (BP) readings taken sequentially leads to an overestimation of patients' "true" level as determined by ambulatory monitoring, UK researchers warn.

And they argue that patients with a large inter-arm difference may simply be experiencing a "white-coat" effect, with BP falling between the two readings being taken, rather than it being an indicator they have peripheral artery disease, as their study found no difference in the extent to which left or right arm readings differ from mean ambulatory BP levels.

"Where a large inter-arm blood pressure difference is detected with sequential measurement, healthcare professionals should re-measure the blood pressure in the original arm," write Una Martin (University of Birmingham) and team in the British Journal of General Practice.

The researchers' study included 784 patients attending a hospital-based hypertension clinic to confirm a diagnosis of hypertension or for further investigation of poor on-treatment hypertension control, drug intolerances, and/or the underlying cause of hypertension.

Data were available for 710 patients (mean age 48.9 years), of whom 279 (39.3%) had a systolic BP difference of 10 mmHg or more between each arm's reading taken 5 minutes apart.

Martin and colleagues first compared patients' highest and lowest clinic arm readings with their mean ambulatory systolic BP monitoring (ABPM) levels over the following 24 hours. They found that the highest reading was a mean of 25.1 mmHg higher than the mean daytime systolic ABPM, while the lowest was 15.5 mmHg higher.

By contrast, they then found the differences between the mean right (20.7 mmHg) and left (19.9 mmHg) readings and daytime systolic ABPM were very similar, suggesting the large disparity between higher arm and ABPM readings is unlikely to be due to arm dominance.

Further analysis showed that disparities with ABPM levels increased with patient age, with an average 0.35 mmHg increase in the difference between either arm and daytime ABPM reading with each year increase in age.

Similar but smaller differences between single diastolic readings and ABPM diastolic readings were observed, the authors note.

The researchers comment: "This study highlights the limitations of single clinic readings, which are not accurate when compared with ABPM or other forms of more prolonged monitoring."

They add that, given most non-specialists are unlikely to have equipment for taking simultaneous measurement in both arms and use of the higher initial reading will significantly overestimate BP, repeated measurements should always be made to guide clinical decisions, as recently recommended in UK guidance.

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