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Blood pressure variability is an important consideration in hypertension trials for determining required sample size and consequently making accurate outcome statements. The Hypertension Prevention Trial was a randomized controlled trial carried out in 1983-1986 in four US clinics on men and women with high normal diastolic blood pressure. This trial provided an opportunity to assess conditions affecting blood pressure measurement variability. Trained blood pressure observers measured systolic and diastolic blood pressures twice, 30 seconds apart, using a random-zero sphygmomanometer. The quality of blood pressure measurements was assessed by computing the variability of the two readings per participant-visit for each blood pressure observer at each study clinic. Other sources of variability investigated included observer digit preference, time of day, and ambient temperature. On the basis of data from this population, it is estimated that the standard deviation of blood pressure values can be reduced by 5% by taking two measurements per participant-visit. An additional reduction of variability can be effected by having the duplicate blood pressure measurements made by different blood pressure observers. In special instances where the range of blood pressure values is very restricted, use of the random-zero sphygmomanometer can increase or decrease the among-participant variability in blood pressure values, depending upon where the distribution of blood pressure values is centered.
Canner et al. (Thu,) studied this question.