Background: The optimal setting and number of blood pressure (BP) measurements that should be used for clinical decision making and quality reporting are uncertain.
Objective: To compare strategies for home or clinic BP measurement and their effect on classifying patients as having BP that was in or out of control.
Design: Secondary analysis of a randomized, controlled trial of strategies to improve hypertension management. (ClinicalTrials.gov registration number: NCT00237692)
Setting: Primary care clinics affiliated with the Durham Veterans Affairs Medical Center.
Patients: 444 veterans with hypertension followed for 18 months.
Measurements: Blood pressure was measured repeatedly by using 3 methods: standardized research BP measurements at 6-month intervals; clinic BP measurements obtained during outpatient visits; and home BP measurements using a monitor that transmitted measurements electronically.
Results: Patients provided 111,181 systolic BP (SBP) measurements (3218 research, 7121 clinic, and 100,842 home measurements) over 18 months. Systolic BP control rates at baseline (mean SBP<140 mm Hg for clinic or research measurement; <135 mm Hg for home measurement) varied substantially, with 28% classified as in control by clinic measurement, 47% by home measurement, and 68% by research measurement. Short-term variability was large and similar across all 3 methods of measurement, with a mean within-patient coefficient of variation of 10% (range, 1% to 24%). Patients could not be classified as having BP that was in or out of control with 80% certainty on the basis of a single clinic SBP measurement from 120 mm Hg to 157 mm Hg. The effect of within-patient variability could be greatly reduced by averaging several measurements, with most benefit accrued at 5 to 6 measurements.
Limitation: The sample was mostly men with a long-standing history of hypertension and was selected on the basis of previous poor BP control.
Conclusion: Physicians who want to have 80% or more certainty that they are correctly classifying patients' BP control should use the average of several measurements. Hypertension quality metrics based on a single clinic measurement potentially misclassify a large proportion of patients.
Primary funding source: U.S. Department of Veterans Affairs Health Services Research and Development Service.