Single blood pressure measurements both in the clinic and at home are not reliable

Research just published in the British Medical Journal reveals that blood pressures measured on single occasions in either the home or clinic results in substantial overdiagnosis and excessive medication. This is obviously a serious concern not just because of the side-effect potential of various antihypertensive medications—suppressing blood pressure lower than necessary results in diminished oxygen delivery to tissues (including the brain). The authors set out to:

"...determine the relative accuracy of clinic measurements and home blood pressure monitoring compared with ambulatory blood pressure monitoring as a reference standard for the diagnosis of hypertension."

Their screening identified seven studies on blood pressure measurements in the clinic and three studies on home measurement that could be directly compared with ambulatory monitoring. What did the data show?

"Compared with ambulatory monitoring thresholds of 135/85 mm Hg, clinic measurements over 140/90 mm Hg had mean sensitivity and specificity of 74.6% and 74.6%, respectively, whereas home measurements over 135/85 mm Hg had mean sensitivity and specificity of 85.7% and 62.4%."

That is a lot of error. Interestingly, at home measurements captured hypertensive readings more sensitively than in the clinic but with more false positives. Overall, measurements done in the clinic and at home got it wrong about a quarter of the time. The authors conclude:

"Neither clinic nor home measurement had sufficient sensitivity or specificity to be recommended as a single diagnostic test. If ambulatory monitoring is taken as the reference standard, then treatment decisions based on clinic or home blood pressure alone might result in substantial overdiagnosis. Ambulatory monitoring before the start of lifelong drug treatment might lead to more appropriate targeting of treatment, particularly around the diagnostic threshold."

The full text of their paper can be read and downloaded here. Coincidentally, another study on the inaccuracy of single blood pressure measurements was just published in the Annals of Internal Medicine. The authors' objective was to...

"...compare strategies for home or clinic BP measurement and their effect on classifying patients as having BP that was in or out of control."

They followed 444 hypertensive veterans for 18 months, comparing standardized research blood pressure measurements every six months, measurements taken in the clinic during outpatient visits, and BP measured at home that transmitted the readings electronically. Their data also showed strong variability:

"Systolic BP control rates at baseline (mean SBP <140 mm Hg for clinic or research measurement; <135 mm Hg for home measurement) varied substantially, with 28% classified as in control by clinic measurement, 47% by home measurement, and 68% by research measurement. Short-term variability was large and similar across all 3 methods of measurement, with a mean within-patient coefficient of variation of 10%. Patients could not be classified as having BP that was in or out of control with 80% certainty on the basis of a single clinic SBP measurement from 120 mm Hg to 157 mm Hg. The effect of within-patient variability could be greatly reduced by averaging several measurements, with most benefit accrued at 5 to 6 measurements."

What to do? To prevent misdiagnosis and especially overtreatment, ambulatory monitoring is the gold standard; but at the least, 5-6 sequential measurements should always be averaged to reduce error. The authors conclude:

"Physicians who want to have 80% or more certainty that they are correctly classifying patients' BP control should use the average of several measurements. Hypertension quality metrics based on a single clinic measurement potentially misclassify a large proportion of patients."