Reproducibility of home, ambulatory, and clinic blood pressure: implications for the design of trials for the assessment of antihypertensive drug efficacy

The aims of this study were to compare the reproducibility of blood pressure (BP) measured in the clinic (CBP), at home (HBP), and by ambulatory monitoring (ABP), and to assess its implications on the accuracy of antihypertensive drug trials. A total of 133 untreated subjects with elevated CBP were assessed with repeated measurements of CBP (five visits within 3 months), HBP (6 workdays within 2 weeks), and ABP (twice, 2 weeks apart). The reproducibility of CBP (one visit), HBP (2 days), and ABP (24 h) was quantified using the SD of differences (SDD) between repeated measurements. The number of subjects required in a comparative trial of two drugs was calculated for each measurement method. We found that HBP provided the lowest SDD values (6.9/4.7 mm Hg, systolic/diastolic, compared with 8.3/5.6 for ABP and 11.0/6.6 for CBP). For a parallel trial aiming to detect a difference in the effect of two drugs of 10 mm Hg systolic BP, 51 subjects would be required when using CBP compared with 29 using ABP and 20 using HBP (73, 53 and 37 subjects, respectively, for the detection of a 5 mm Hg difference in diastolic BP). The study shows that HBP seems to have superior reproducibility compared with both CBP and ABP. In addition, HBP can improve the accuracy of antihypertensive drug trials, thereby reducing the sample size required.