Health Care

March 26, 2014

Professional Opinion: Is the store blood pressure machine accurate?

This week, Dr. Jay Kalan, a cardiologist with Hilton Head Hospital, talks blood pressure machines and how to get the most accurate reading.

Question. How accurate are the free blood pressure machines in stores? How can you get the most accurate reading on a free machine? Are home blood pressure machines more accurate? How much will my blood pressure vary throughout the day, and when is the best time to check it?

Answer. Free blood pressure machines are available in most pharmacies and are used by many people. Accuracy of these machines is very variable.

Machines are most accurate when first installed, or after recalibration and testing by the company. This testing is typically done about twice per year, or if a problem is reported by the store. (The machines are not typically owned by the store.)

Accuracy is also directly related to arm size. The machines are most accurate for average-sized arms, and less accurate for people with small- or large-sized arms, as reported in a study by Mayo Clinic.

Lastly, there is more variation between readings than is acceptable when multiple readings are taken. To get the most accurate reading, use the machine only if you have an average- or mildly large-sized arm. Sit a minute and rest before taking the blood pressure. If you get a very abnormal reading, measure the pressure again.

Home blood pressure machines appear to be more accurate, assuming the cuff size is appropriate to a person's arm size. Most cuffs have a size range printed on the inner aspect to confirm that you are using the correct size for your arm.

If the accuracy of your home cuff is in question, it should be brought to your health care provider's office to compare results against their machine.

The brand of home monitor does not appear to be important. All the locally available brands appear equally as accurate.

The American Heart Association recommends a home cuff that measures above the elbow, rather than at the wrist. The wrist monitors are less accurate due to the smaller size of the artery there, and are more positional.

They are, however, appropriate for people with very large upper arms, where the standard cuffs would be too small, or in people who find blood pressure measurements painful when taken on their upper arm. Also, it is best if the cuff automatically inflates, and automatically records the pressure.

Blood pressure does vary through the day. It is lower at night, while asleep, and higher during waking hours. Blood pressure readings should be taken after five minutes of sitting, rest.

Normal pressure averages 120/80, with hypertension defined as a blood pressure over 140/90. Any activity or exercise raises the pressure.

After exercise, the degree of change can range from 10 to 60 mmHg. Blood pressure then drops down even lower than before (hence the benefit of regular exercise).

Blood pressure needs to be normal throughout the day, so when you measure isn't as important as measuring it at about the same time of day, especially if medications are being adjusted.

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