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We were interested to read Parker and Glasziou's assessment of previous hypertension guidelines and their advice on the measurement of both arms, to identify an inter-arm difference.1 We have also researched this subject,2 and found that the advice to measure both arms can be traced back, almost verbatim, over 70 years.3 The ESH 2007 guidelines, however, took a significant step in attributing an inter-arm difference to peripheral vascular disease for the first time.4 Unfortunately, the weight of evidence supporting this statement was not assessed in that guideline, exemplifying Parker and Glasziou's argument. We have sought clarification of this evidence, but requests to the guideline's authors have not received a response. Consequently, we have been conducting our own systematic review of the evidence associating an inter-arm difference with peripheral vascular disease. Preliminary results of our meta-analysis suggest a significant association of a systolic inter-arm difference >10 mmHg or >15 mmHg with peripheral vascular disease (OR = 5.25, 95% CI = 2.85 to 9.70 and OR = 6.46, 95% CI = 4.85 to 8.61 respectively, both P<0.001). Seemingly this is exactly the sort of evidence base to justify measuring both arms, but all studies included in the analysis were of populations at existing high vascular risk, for example, referrals to angiography services. We cannot discover an evidence base that permits extrapolation of the guideline statement to the general population of which it is aimed.
Parker and Glasziou also raise the important issue of how to measure an inter-arm difference. We have found that prevalence of an inter-arm difference is over-estimated without a robust measurement technique.2 While this is of epidemiological importance we have found repeated simultaneous measurements to be a barrier to recruitment in primary care5 and this approach has been criticised as impractical.6 To overcome this we have compared the use of a single sequential pair of measurements to our gold standard simultaneous technique in 187 subjects in primary care with type 2 diabetes. Preliminary findings in 187 subjects have shown a high negative-predictive value of 0.97 in excluding a systolic inter-arm difference >10 mmHg.7 Consequently, the vast majority of subjects who do not have an inter-arm difference can be identified within a single consultation, and only the 10–20% remaining will need further assessment. The validity of this approach, and the clinical implications of detecting an inter-arm difference in subjects at low cardiovascular risk, both require further study.