MONDAY, June 20 (HealthDay News) -- Evaluating how well a person responds to medication meant to lower blood pressure requires multiple readings, new research suggests.

Blood pressure often spikes at doctor appointments, a condition known as "white coat" hypertension, so readings should also be taken by patients at home, said the study's lead author, Dr. Benjamin Powers, assistant professor of medicine at Duke University Medical Center.

''Collecting just a few, five or six, blood pressure readings from home will help your doctor make much better decisions about whether your blood pressure is in or out of control," Powers said.

For the study, Powers and his colleagues evaluated 444 U.S. veterans with high blood pressure, about 90 percent of them men with an average age of 64. All had been diagnosed about 10 years earlier. Their blood pressure was considered uncontrolled, even though most took multiple blood pressure-lowering medications. Untreated, high blood pressure can lead to stroke.

The study compared blood pressure readings taken in three settings -- at home, in a doctor's office, and at a clinical research setting -- at the study start and again at 6, 12 and 18 months. The findings are published in the June 21 issue of the Annals of Internal Medicine.

The measurements varied widely, Powers found. "Only a third of them were consistently categorized as in or out of control by all three measures," he said.

That points to the importance of getting multiple measures from different settings, said Powers, who is also with the Durham Veterans Affairs Medical Center. Using just one reading as the basis for prescribing or adjusting blood pressure medication could endanger patients who suffer from "white coat" hypertension, the study authors said.

As many as six readings were needed to obtain the best estimate of true blood pressure, the authors said, adding that doctors should average the results.

Home monitoring of blood pressure has gained respect in recent years, Powers said. About 43 percent of people with high blood pressure use home monitors, according to background information in the study. Powers and his colleagues said their results support recent calls for reimbursement of home blood pressure monitoring.

"I think patients should expect their doctors to make decisions based on home blood pressure [in addition to other measures]," he said.

Those who buy a home blood pressure monitor should get it validated for accuracy at their doctor's office, Powers added.

Patients should be advised to measure their blood pressure at different times of the day, and to sit and relax for five minutes before taking it, Powers explained. Variations are to be expected, he noted.

According to Dr. Joseph Diamond, director of nuclear cardiology at Long Island Jewish Medical Center in Hyde Park, N.Y., blood pressure readings are perhaps "the most frequent assessment that results in a change of medical therapy."

Even so, the procedure is frequently flawed because of poor measurement technique or insufficient measurements, he said.

"This study confirms the need to improve both measurement technique and the number of measurements obtained so that a therapeutic decision to start or change medication is based on more accurate information," said Diamond.

Writing in an accompanying editorial of the journal, Dr. Lawrence J. Appel and colleagues from Johns Hopkins University said the findings point to a need for regulation of blood pressure screening. "It is time to get serious about BP measurement," they wrote.

More information

To learn more about home blood pressure monitoring, visit the British Hypertension Society.