Variability of office, 24-hour ambulatory, and self-monitored blood pressure measurements

BACKGROUND: The diagnosis of hypertension is difficult when faced with several different blood pressure measurements in an individual. Using the average of several office measurements is recommended, although considerable uncertainty remains. Twenty-four-hour ambulatory monitoring is often considered the gold standard, but self-monitoring of blood pressure has been proposed as a superior method. AIM: Determination of within-individual variability of blood pressure measured in the office, by ambulatory monitoring, and by a week of self-monitoring. DESIGN OF STUDY: Retrospective analysis of a clinical trial of 163 subjects. METHOD: Within-patient variability of office and ambulatory blood pressure was determined from measurements at 0 and 6 weeks. Subjects had performed self-monitoring of blood pressure twice each morning and evening, for at least 6 weeks; variability was determined from the means of week 1 and week 6. RESULTS: The within-individual coefficients of variation (CVs) for systolic blood pressure were: office, 8.6%; ambulatory, 5.5%; self, 4.2%. Equivalent values for diastolic blood pressure were 8.6%, 4.9%, and 3.9%. CVs tended to be lower with longer self-monitoring duration, and higher with longer intervals between self-monitoring. CONCLUSION: Office blood pressure is impractical for precise assessment, as 10-13 measurements are required to give the accuracy required for rational titration of antihypertensive drugs. Twenty-four-hour ambulatory monitoring is better than a single office measurement, but considerable uncertainty remains around the estimate. A week of self-monitoring appears to be the most accurate method of measuring blood pressure, but remains imperfect. Further research may identify superior self-monitoring schedules. Given the inherent accuracy in blood pressure measurement, the importance of considering overall cardiovascular risk is emphasised.