Manual blood pressure measurements may not be as accurate as the automatic devices that take multiple measurements.

If your doctor still uses a manual device to check your blood pressure, you risk being misdiagnosed, experts say.

This could lead to unnecessary treatment with medication for high blood pressure, not to mention dealing with the side effects and expense of those drugs.

In a new study published in the journal Canadian Family Physician, researchers estimated that about 20 percent of Canadians being treated for high blood pressure, or hypertension, don’t actually need medication because their blood pressure was measured incorrectly.

Researchers say this is due to more than half of family doctors in Canada still using manual devices to measure blood pressure.

Read more: How to understand blood pressure readings »

Measuring someone’s blood pressure manually seems like a fairly straightforward procedure — put on the cuff, inflate it, and read the dial as the cuff deflates.

A lot can go wrong, though.

“It seems like blood pressure is easy to measure, but almost no one does it correctly,” said Dr. Sarah Woolsey, a family physician, and medical director with HealthInsight Utah.

Using the wrong size cuff can throw off the numbers. It can also happen if a person is fidgeting while having their blood pressure checked, or checking someone’s blood pressure right after they come into the office.

Even just being in the doctor’s office can cause a person’s blood pressure to increase. This kind of “white-coat hypertension” is surprisingly common.

“The big message for us is that ‘white-coat hypertension’ is prevalent, and over-treatment may be prevalent and may lead to side effects of medication,” Woolsey told Healthline.

To avoid these types of errors, the Canadian Hypertension Education Program Guidelines recommend that doctors use automated blood pressure monitoring in the office.

“Clinicians should use automatic devices. They are more expensive but more precise because they take several measurements,” Janusz Kaczorowski, PhD, a professor in the Department of Family and Emergency Medicine at Université de Montréal, and lead author of the Canadian study, said in a press release.

Automated blood pressure monitoring can also be done while the person is alone, which reduces some of the white-coat hypertension.

Does that mean manual measurements are always wrong? No, but their accuracy depends on how they’re done.

“Manual measurement is acceptable if it’s properly done, but that’s often not the case,” said Kaczorowski. “To take blood pressure the right way, a 12- to 15-minute period is required.”

This may not work with the typical 10-minute visit to a family doctor.

Read more: Automated vs. manual blood pressure readings »

The U.S. Preventive Services Task Force (USPSTF) offers similar guidelines for blood pressure screening.

The blood pressure measurement that happens at a routine doctor’s visit is an initial screening.

If someone’s blood pressure is elevated, then a more thorough check is needed to properly diagnose high blood pressure.

This can be done in several ways.

“The gold standard is the 24-hour ambulatory monitor,” Woolsey told Healthline.

If that isn’t available, Woolsey said the guidelines “suggest multiple measurements of blood pressure — and they give you the option of an automated office blood pressure, or out-of-office monitoring, or multiple in-office monitoring.”

Ambulatory blood pressure monitoring devices are small, portable machines that measure a person’s blood pressure at regular intervals over 12 to 24 hours. This gives a more accurate picture of a person’s “real” blood pressure level.

However, these devices may be hard to come by, especially in rural areas.

A recent paper published by Woolsey and her colleagues in the Journal of the American Board of Family Medicine (JABFM) found that only a quarter of 123 primary care clinics in Utah that responded to a survey had access to ambulatory monitoring.

Researchers also discovered that clinics were not following all of the USPSTF guidelines.

At the high end, 93 percent of the clinics reported using an appropriately sized arm cuff.

But only 57 percent of clinics waited at least five minutes after the patient arrived at the office before checking blood pressure.

Woolsey thinks the situation may be similar in clinics throughout the United States.

Because of the lack of ambulatory devices in many areas and the challenges with manual blood pressure monitoring, automated devices may have an even bigger role to play in clinics.

“We think the automated office blood pressure will be a good solution, especially for our smaller cities and smaller towns,” said Woolsey.

Read more: Lowering your blood pressure could save your life »

People with high blood pressure often don’t feel sick until their pressure is really high or they develop another problem such as kidney disease, heart attack, or stroke.

That’s why high blood pressure is known as a “silent killer.”

Home blood pressure monitoring can help people track their condition in between doctor visits.

This can be done using standard manual blood pressure cuffs or automated home devices.

For this to work well, though, people need to be properly trained — another area of improvement identified by Woolsey and her colleagues.

Slightly more than a third of the Utah clinics provided instructions to people on how to measure blood pressure accurately at home.

But once people know how to check their own blood pressure correctly, home monitoring can help them in more ways than one.

“I personally aspire to a day where we have diabetes without doctors, and hypertension treatment without doctors, because patients are so enabled in self-care,” said Woolsey. “Self-monitoring programs are good for patient engagement and patient self-awareness of their own conditions.”