Self-monitoring of blood pressure at home: how many measurements are needed?

Objective To determine the minimum number of self-measurements of blood pressure at home (HBP) necessary to provide the maximum clinically important benefit. Methods Hypertensive patients were randomly allocated to monitor HBP for 2 weeks (6 work days, duplicate measurements, twice daily) or ambulatory blood pressure for 24 h. The alternative measurement was then performed. Clinic blood pressure was measured at the beginning and the end of the study. Criteria for reliability of HBP were the stabilization of mean HBP, its variability (SD) and the correlation coefficient r for relationship of HBP with ambulatory blood pressure. The reproducibility of HBP was quantified using test-re-test correlations and the SD of differences between average HBP values of different days. Results We studied 189 patients (79 being administered stable antihypertensive treatment). Average HBP (137.5 +/- 16.2/85.9 +/- 9.9 mmHg) was lower than average clinic blood pressure (P < 0.001) and higher than 24 h and night-time ambulatory brood pressures (P < 0.001), There was no difference between HBP and daytime ambulatory blood pressure. On day I HBP was higher than it was on each of days 2-6, with no difference among days 2-6. When data for the initial day for monitoring of HBP were excluded from analysis, average HBP was reduced. Only a modest improvement in the reliability of HBP on day 2 (reductions in mean HBP and its SD and an increase in r with ambulatory blood pressure) was achieved by averaging more readings taken on succeeding days. At least two monitoring days were needed for the reproducibility of HBP to be superior to that of clinic blood pressure. Conclusions These results suggest that determining average HBP of the second and third work days, is the minimum programme that provides a reliable estimate of HBP. (C) 1998 Lippincott-Raven Publishers.