A doctor's equipment may give the most accurate PR measurement, but you can bring in your home machine for comparison.

RODALE NEWS, EMMAUS, PA—Just this year, researchers found that taking your blood pressure at home yields truer results than the ones you'd generate at the doctor's office. (The idea is, you're calmer at home.) However, a new study published in The American Journal of Cardiology found you may not want to completely rely on automatic blood pressure monitors' results. Researchers from the University of Southern California found some startling discrepancies between the newer automatic blood pressure readers and the more traditional mercury manometer/stethoscope combo. The automatic monitors consistently produced lower results, meaning you could have high blood pressure, or pre-hypertension, and think your health is not at risk.

And getting an accurate reading is important. "If you control high blood pressure, you reduce risk of heart failure, stroke, heart attack, and kidney disease," says study coauthor Robert Kloner, MD, professor of medicine at the University of Southern California Keck School of Medicine, and director of Research at the Heart Institute of Good Samaritan Hospital in Los Angeles.

THE DETAILS: Researchers looked at 337 patients seen in routine cardiology office visits who received blood pressure measurements with both the more old-fashioned mercury manometer technique or the increasingly popular automatic blood pressure monitor. The systolic readings (the top number), on average, were about 2 mm higher when using the mercury manometer method; for the diastolic reading (the lower number), it read about 1.3 mm higher. That might not seem like much, but Dr. Kloner notes that there were great variations, some of which could be the difference between being labeled "healthy" or hypertensive, or worse yet, could be a warning sign for a heart attack or stroke. For instance, when looking at systolic numbers, in 5 percent of patients, the discrepancy spanned five to 10 points; in 7 percent, researchers noted a 10- to 15-point discrepancy. In 4 percent of participants, the difference between the readings on the two different types of machines was 15 to 20 points!

It's important to note that in the group of participants younger than 65, discrepancies occurred 10 percent of the time. However, in the older adults, that number jumped to nearly 30 percent. One theory as to why involves the idea that older people's arteries are stiffer, which could lead to less-accurate blood pressure readings.