Originally Published: 07 AUG 23 11:53 ET

By Giri Viswanathan, CNN

(CNN) — Dr. Georges Benjamin was shocked when he got his vitals checked before undergoing cataract surgery. His blood pressure, it seemed, had skyrocketed. But after a second check from the blood pressure machine, and taking his own blood pressure at home, it had returned to much more acceptable levels.

The difference, it turns out, had nothing to do with his heart health.

At the hospital, his first blood pressure cuff had been too small, and it was changed only at his request. For Benjamin, the executive director of the American Public Health Association and a self-described “big guy,” an undersized blood pressure cuff had dramatically thrown off his readings.

Now, a new clinical trial highlights how much an improperly sized blood pressure cuff can distort blood pressure readings taken by automated blood pressure devices. The study, published on Monday in the journal JAMA Internal Medicine, found that using standard, regular-sized blood pressure cuffs resulted in “strikingly inaccurate” readings if a patient required differently sized ones, particularly for participants who fit larger cuffs.

Automatic blood pressure devices are commonly used to diagnose hypertension, the leading cause of heart disease worldwide. The cuffs, or the sleeves that wrap around a person’s upper arm, typically come in four sizes ranging from small to extra-large, depending on the size of the person’s arm.

But frequently, only regular-sized cuffs are used to take someone’s blood pressure, even though clinical guidelines recommend using cuffs suited for a person’s arm circumference. Sometimes, experts say, that’s because switching out cuff sizes can be inconvenient for health care providers: If a clinic is busy or short-staffed, personnel might not have the training or time to find the right cuff for each patient. Cuffs might also be broken without replacement, said Benjamin, who was not involved in the study.

“I think it’s the lack of attention to detail,” he said. “Make sure that there are enough cuffs around that are working and in proper order and someone paying attention and checking that equipment on a regular basis.”

Meanwhile, at community health sites and low-resource clinics, staff might only have the regular-sized cuff available to use when recording blood pressure, Benjamin said. Even widely used at-home blood pressure monitors often include only one cuff size.

Either way, experts say, a one-size-fits-all approach can skew blood pressure readings, interfering with core metrics that doctors use to monitor and treat heart conditions.

“Many of the children that I get referred to for hypertension often don’t have their blood pressure measured with a cuff that is appropriately sized for them,” said Dr. Tammy Brady, a study author and an associate professor of pediatrics at Johns Hopkins University. “I find much lower blood pressures when I measured their blood pressure compared to the referral blood pressures.”

In the randomized trial, 195 people in Baltimore had their right arms measured by trained health care staff to fit an appropriate-sized cuff. After instructing the participants to walk around for two minutes, the researchers measured their blood pressure using three different cuff sizes in random order: the regular, standard size; an appropriate, fitted size; and a third size too small or too large.

For patients who fit smaller cuffs, the study found, using a regular cuff size resulted in consistently low blood pressure readings. Meanwhile, individuals that required large or extra-large cuff sizes received high blood pressure measurements when using regular cuffs — with bigger errors, on average, than people who needed smaller cuffs.

In other words, using a smaller-than-recommended cuff size led to artificially high blood pressure numbers, while using a larger cuff size resulted in falsely low readings. Those effects did not change depending on a person’s existing blood pressure or obesity status, according to the report.

“We were shocked at how big of a difference there was,” Brady told CNN. “I didn’t expect the magnitude that we found.”

For instance, individuals who should use an extra-large blood pressure cuff received, on average, a systolic blood pressure reading that was 13.5% too high when using a cuff that was two sizes too small. That’s a difference, according to Brady, that can change a doctor’s hypertension diagnosis entirely.

Even if a health care provider is “trying to do the right thing” and select an appropriate blood pressure cuff, she added, an incorrect measurement that results in a different cuff size can still have serious implications.

In the study, the average blood pressure for someone who needed an extra-large cuff was 125/80: slightly elevated, and potentially pre-hypertensive, but not highly concerning. When those patients used an undersized “regular” cuff, however, their average blood pressure was 144/87 — a level that correlates with stage two hypertension. “It can have really significant clinical implications.”

However, there are a few important steps that health care providers and patients can take to ensure that blood pressure readings stay accurate, experts agree.

Ensuring that facilities that screen blood pressure have a full range of sizes is key. But in Brady’s experience, even when clinics do have a variety of cuffs, they might not be readily available: an assistant might have to retrieve it from a password-protected medical closet, eating up valuable time when screening a patient.

“A person in triage should not have to walk … to get the right cuff because that’s just going to slow things down and make it less likely that they’ll actually use the right cuff,” she said.

Educating both patients and providers could also help, Brady said. People who are administering blood pressure tests — including workers at community health events, who may not have medical training — should be trained to understand the importance of appropriate cuff selection.

And if patients are buying home blood pressure machines, she said, they “need to know that getting the right cuff size is important,” After all, if a patient using a small cuff size reports high blood pressure to their doctors, it could affect their care.

For Benjamin, if patients are taught to use self-run blood pressure devices, they might be a tool that people use to flag issues and follow up with their health care providers. Patients with high blood pressure can be asymptomatic, he warned, and repeated high readings might be a sign to follow up with a doctor.

But Brady warned against using blood pressure kiosks like those commonly found in grocery stores, since they may not be tested or validated for accuracy. If the kiosk’s cuff is the wrong size, there’s no guarantee that the measurement is accurate: many users might put their arm inside and only get a reading, for instance, from their forearm.

“Most people take for granted the blood pressure,” Brady said. “They assume that the machine is very smart and it’s just going to give you the right value, but there’s a lot that that depends on the operator doing things properly.”