NEW YORK (Reuters Health) – The average of five or six blood pressure measurements are needed in order to classify patients with reasonable certainty as having blood pressure in or out of control, according to a new study published in the Annals of Internal Medicine for June 21.
“Current decisions about medication therapy are often made on the basis of 1 or 2 clinic measurements,” note Dr. Benjamin J. Powers, with the Durham Veterans Affairs Medical Center in North Carolina, and colleagues. The current findings, they say “suggest that this could be substantially improved for patients with a history of elevated BP measurements when decisions are based on the average of several measurements, regardless of the setting.”
The authors point out that there is no consensus on the setting or number of blood pressure measurements that should be used for classifying patients and making treatment decisions. While home BP monitors are widely available and accurate, they continue, treatment decisions are most often based on clinic measurements.
To look into the accuracy of different measurement strategies for determining a patient’s true BP, the authors performed a secondary analysis of data from a trial aimed at improving hypertension management.
During the trial, 444 veterans (92% male) with hypertension provided 111,181 systolic BP measurements over an 18 month period. These included 3218 standardized measurements obtained in a research setting, 7121 measurements made in the clinic, and 11359,842 measurements with a home monitor that transmitted readings electronically.
“We restricted all analyses to systolic BP because of its greater importance in cardiovascular events and treatment decision making,” the investigators explain.
Blood pressure was considered under control with a systolic BP reading of less than 140 mm Hg for clinic or research measurements, and less than 135 mm Hg for home measurements. The investigators found that control rates varied substantially for different methods of measurement: 28% according to clinic measurements, 47% based on home measurements, and 68% with research readings.
Within-patient variability was large and similar for all three methods of measurement, the mean coefficient of variation being 10%, according to the report. This meant that single measurements were not accurate in classifying BP control unless they were fairly extreme.
For example, a single clinic measurement between 120 and 157 mm Hg could not classify patients as having BP in or out of control with 80% certainty. In the home setting, the corresponding range of uncertainty for a single measurement was 123 to 153 mm Hg.
However, further analysis indicated that accurate classification with greater than 80% probability could be obtained by averaging several readings – five, in the case of home measuring, Dr. Powers and colleagues report.
Summing up, they conclude, “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.”
Ann Intern Med. 2011;154:781-788.