Clinical Question: How many blood pressure readings are necessary to accurately diagnose hypertension?
Bottom Line: Several blood pressure measurements, taken over time, should be averaged to obtain a patient's "true" blood pressure. Using a single measurement to determine whether blood pressure is controlled will yield false results more than 20% of the time. This study used an automated method to measure blood pressure; results are likely even worse when blood pressure is measured manually (BMJ 2011;342:d286). (Level of Evidence: 1b)
Reference: Powers BJ, Olsen MK, Smith VA, Woolson RF, Bosworth HB, Oddone EZ. Measuring blood pressure for decision making and quality reporting: Where and how many measures? Ann Intern Med 2011;154(12):781-788.
Study Design: Cohort (prospective)
Funding Source: Government
Setting: Outpatient (any)
Synopsis: There are many things that can falsely elevate blood pressure -- the "white coat effect," poor technique, bad equipment, and so forth. In addition, blood pressure varies from moment to moment in an individual. In this study, the investigators evaluated repeated blood pressure measurements over time in a hypertension study, comparing results obtained in normal clinical practice, home measurement results, and measurements as part of the research protocol. The study was conducted in 444 men undergoing hypertension treatment (average age = 64 years) who had a total of 111,181 blood pressure determinations over 18 months. All patients had blood pressures measured by all 3 methods, though it doesn't appear that results were obtained in a masked manner. A patient's "true" blood pressure was calculated by modeling the average blood pressure over time with intrapatient variability. All 3 methods used automated devices. Over time, the average variation in systolic blood pressure in an individual was 10%, regardless of method of measurement. The research assessment, which probably had the closest adherence to protocol, resulted in patients being classified as having blood pressure controlled at baseline 68% of the time, compared with 47% with home measurement and 28% with clinic measurement. A single blood pressure measurement using any method was quite inaccurate when systolic blood pressure was between 120 mmHg and 157 mmHg. To obtain at least 80% certainty of an accurate blood pressure, several blood pressure readings must be taken over time (weeks to months) and averaged.