More than one blood pressure measurement is necessary for accuracy

Accurate blood pressure assessment requires multiple measures regardless of whether it is measured at home, in a clinic, or in a research setting, according to findings from a study of more than 400 veterans. In a secondary analysis of a randomized trial, within-patient variability in blood pressure readings decreased and certainty about the true reading was enhanced as the number of measurements increased, according to Benjamin Powers, MD, of the Durham VA Medical Center in North Carolina, and colleagues.  "In hypertension, simple changes in the setting and number of blood pressure measurements used for decision making could greatly enhance the personalization of care," they wrote in the June 21 issue of Annals of Internal Medicine. "If providers are supposed to rely more on averaged measurements, new ways of capturing and presenting these data at the point of care are needed," Powers and his colleagues added.  "Calculated averages from home monitors, blood pressure control charts that visually display the signal–noise relationship, or personalized algorithms that account for each patient's own variability may improve the interpretation of blood pressure and facilitate more informed and individual decisions." Clinicians often cite uncertainty about a patient's true blood pressure based on clinic measurements as a common reason for not changing therapy, according to the researchers. To look at the certainty with which a patient's blood pressure can be determined using various methods, Powers and his colleagues performed a secondary analysis of the Hypertension Intervention Nurse Telemedicine Study (HINTS), which was conducted in primary care clinics affiliated with the Durham VA Medical Center.  The current analysis included 444 veterans with hypertension. Their mean age was 64, most were men (92%), and three-quarters had hypertension for at least 10 years.  Blood pressure was measured repeatedly throughout the 18-months study in three ways -- standardized study blood pressure readings at six-month intervals, clinic readings during outpatient visits, and home readings using a monitor that transmitted measurements electronically. Rates of systolic blood pressure control (a mean of less than 140 mm Hg for clinic or research measurements and less than 135 mm Hg for home measurements) varied greatly between the strategies, with 28%, 47%, and 68% considered under control by clinic, home, and research measurements, respectively. Half of the patients had a mean clinic reading that was 10 mm Hg greater than their mean home reading. Within-patient variability was substantial as well, with a mean coefficient of variation of 10% across all three randomized groups.  A single systolic blood pressure reading from 120 to 157 mm Hg could not be used to classify blood pressure control with 80% certainty. However, within-patient variation decreased and certainty about the true blood pressure value improved as the number of measurements increased. The largest improvement occurred when a second reading was added, with little additional precision beyond four to six measurements. The findings show that hypertension quality metrics based on a single clinic measurements -- as practiced within the Healthcare Effectiveness Data and Information Set of the National Committee for Quality Assurance -- may misclassify a large proportion of patients in terms of blood pressure control. The study "highlights the benefits of recording and averaging high-quality blood pressure measurements across several visits," according to Lawrence Appel, MD, MPH, of Johns Hopkins University, and colleagues. Although the cost of getting repeated measurements across clinic visits is a concern, they wrote in an accompanying editorial, home blood pressure readings improve precision to a similar extent. "Hence, a benefit of home blood pressure measurement is frequent... readings that can be averaged and can potentially obviate the need for repeated clinic visits," they wrote. Appel and his colleagues advocated "a regulatory approach in which professional organizations include blood pressure measurement as a performance metric" and pointed out that health-information technologies could be used to automatically calculate average blood pressure from previous visits. "It is time to get serious about blood pressure measurement," they wrote.  Powers and his colleagues noted some limitations of the study, including the use of patients who were mostly male, had a long-standing history of hypertension, and had a history of poor blood pressure control.  The study was supported by a grant from the U.S. Department of Veterans Affairs Health Services Research and Development Service. Powers is supported by a U.S. Department of Veterans Affairs Career Development Award. One of his co-authors is supported by an Established Investigator Award from the American Heart Association and a U.S. Department of Veterans Affairs Health Services Research and Development Service Career Scientist Award.  Powers and two of his co-authors reported receiving grant money through their institution from the U.S. Department of Veterans Affairs Health Services Research and Development Service. One of the other study authors reported being employed by the Medical University of South Carolina and the University of Iowa.

Primary source: Annals of Internal Medicine Source reference: Powers B, et al "Measuring blood pressure for decision making and quality reporting: where and how many measures?" Ann Intern Med 2011; 154: 781-788. Additional source: Annals of Internal Medicine Source reference: Appel L, et al "Improving the measurement of blood pressure: Is it time for regulated standards?" Ann Intern Med 2011; 154: 838-839.