Drugstore Blood Pressure Machines May Not Be Dependable

From the WebMD Archives

May 18, 2000 -- For people who routinely depend on drugstore machines to monitor their blood pressure, a Florida physician has a bit of advice -- don't -- especially if you are larger or smaller than average. When it comes to automated blood pressure machines, one-size arm cuff doesn't really fit all.

In a study reported in the May issue of The Journal of Family Practice, researchers found a big difference among blood pressure readings received by different-sized people who visited 25 pharmacies in Central Florida.

"We found that automated blood pressure machines ... did not meet accepted standards of accuracy and reliability of measurement," writes lead researcher Daniel J. Van Durme, MD. "Although further study in this area may be useful to strengthen this recommendation, we recommend that patients not rely on the results of in-store automated blood pressure devices."

Blood pressure is the force of the blood pushing against the walls of arteries each time the heart beats. When the heart contracts and pumps blood, the pressure it exerts on blood vessel walls is called systolic pressure. The pressure on the vessel walls between beats is called diastolic pressure. Blood pressure is always given as these two numbers, the systolic and diastolic pressures. Usually, they are written one above the other, such as 120/80 mm Hg, with the top number the systolic pressure, and the bottom the diastolic pressure.

For people with a small arm size, systolic measurements were on average 10 mm Hg higher than readings taken right after by a clinician with a portable mercury manometer, another instrument used to measure blood pressure. Diastolic pressure readings averaged 9 mm Hg higher.

For people with a larger than normal arm, diastolic readings averaged 8.3 mm Hg lower than the clinician's measurements, but systolic readings were not that different.

Readings for the person with an average arm diameter were not significantly different between the store readings and the clinician's test, but Van Durme says average-sized people should not depend on the accuracy of the machines.

"Even patients with a medium-sized arm can expect significant and unacceptable variability in blood pressure readings," Van Durme writes. Van Durme is a physician in the department of family medicine and the division of cancer control at the University of South Florida.

Vita-Stat, a division of Spacelabs Medical, manufactured 23 of the 25 machines tested. The other two machines were Health Clinic Cardio-Analysis devices.

Karyn Beckley, vice president of corporate administrative services at Vita-Stat, tells WebMD that the Association for the Advancement of Medical Instrumentation (AAMI) publishes standards for the accuracy of blood pressure measuring devices. To meet AAMI standards, readings from devices must consistently be within plus or minus 5 mm Hg. "[Our] machines all meet the AAMI standards," she says.

Since 1993, the American College of Physicians says patients should not rely on blood pressure readings from self-monitoring blood pressure devices, and Beckley agrees the machines are no substitute for a doctor's care. "It is a way to check on your blood pressure for free in between doctor's visits," she says. "Still, our machines take 200 million readings per year nationwide, and we have had a number of times consumers have been alerted of a problem."

Van Durme says the manufacturers of the devices should monitor the accuracy and reliability of the devices frequently and make maintenance information readily available. Beckley says each individual machine is calibrated on a scheduled basis. She also says Vita-Stat has a 24-hour service organization nationwide to handle calls about defective machines.

"Anytime a person believes a machine is inaccurate, they just need to tell the pharmacist and he can report the problem to us," Beckley says.

Vital Information:

  • A new study shows that blood pressure machines found in drugstores are often inaccurate, especially if a person's arm is larger or smaller than average.
  • A spokeswoman for one of the manufacturers argues that the machines all meet industry standards and help patients monitor blood pressure between doctor visits.
  • Since 1993, the American College of Physicians has advised against relying on self-monitoring blood pressure devices.