Measuring Blood Pressure for Decision Making and Quality Reporting: Where and How Many Measures?
Is clinic or home measurement of blood pressure more accurate in classifying patients’ blood pressure control, and what is the optimal number of measurements?
The authors analyzed data from the Hypertension Intervention Nurse Telemedicine Study (HINTS), a randomized, controlled, 18-month study conducted in the Durham Veteran’s Affairs Medical Center, designed to evaluate the effect of self-management, versus physician-directed, versus both in the management of hypertension, when administered by a nurse via telephone. During this study, subjects had blood pressure measurements recorded at baseline, 6, 12, and 18 months, using a BpTRU digital blood pressure monitor (BpTRU Medical Devices, Coquitlam, British Columbia, Canada), averaging two measurements 5 minutes apart. Home blood pressure measurements were provided at least three times per week. Clinic blood pressure measurements were performed at routine clinic visits using automated devices (IVAC San Diego, CA). The percentage of subjects with blood pressure in control by each method of measurement was reported, as well as a coefficient of variation, and the effect of multiple measurements.
Data for 444 veterans with hypertension, providing 111,181 systolic blood pressure measurements (systolic blood pressure) over 18 months, were analyzed (3,218 research, 7,121 clinic, and 100,842 home measurements). There was substantial variation with all three methods, resulting in 28% classified as “in control” by clinic measurement, 47% by home measurement, and 68% by research measurement. The mean within-patient coefficient of variation was 10% (range, 1-24%), and was similar across all three methods. A single clinic systolic blood pressure could not classify subjects as in or out of control with 80% certainty, unless the measurement was 120 mm Hg or less to correctly classify a patient as in control, or higher than 157 mm Hg to correctly classify a patient as out of control. The effect of within-patient variability was greatly reduced by averaging multiple measurements, with maximal benefit at 5-6 repeated measurements.
The authors concluded that clinicians who want to have 80% or more certainty that they are correctly classifying patients’ blood pressure control should use an average of several measurements, and that quality metrics or clinical decisions based on a single clinic blood pressure measurement would potentially misclassify a large proportion of patients.
The phenomenon of 'white coat hypertension,' and its potential to misclassify many patients’ blood pressure control, is well-known. The current study suggests that, by simply recording three blood pressure measurements at home per week, the method of home measurement would almost double the number of patients identified as having their blood pressure in control, compared with a single clinic blood pressure measurement. Although this study suggests similar variability among home and clinic blood pressure measurements, and a similar need for repeated measurements with either method, it is much easier to obtain the five or six measurements necessary to maximize precision with home blood pressure measurements. The hypertension community, represented by the American Society of Hypertension and the European Society of Hypertension, has long recognized the inferiority of clinic blood pressure measurements, especially when not repeated, and has called for greater use of home blood pressure monitoring in position statements. The data from the current study strongly support the superiority of repeated home blood pressure measurements over single or even duplicated clinic blood pressure measurements for correctly classifying blood pressure control in our patients.
Keywords: Decision Making, Canada, British Columbia, Veterans, Telemedicine, Blood Pressure Monitoring, Ambulatory, Sphygmomanometers, Cardiovascular Diseases, White Coat Hypertension, Blood Pressure Determination, Hypertension, United States
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