Apple Watch EKG not as accurate for younger people, physician says

Apple's latest smartwatch feature, which enables wearers to take an electrocardiogram, may not be as effective at detecting abnormal heart rhythms in younger people as it is for older wearers, according to Daniel Yazdi, MD, a second-year internal medicine resident at Boston-based Brigham and Women's Hospital.

In an op-ed for STAT, Dr. Yazdi said he's excited about "this new frontier in digital health," while also "cautious about its implications."

"Diagnostic tests like this one are rarely perfect," he writes. "They usually generate false positives: incorrectly diagnosing a condition in people who do not have it."

That's a common concern with the Apple Watch's EKG feature: When Apple announced the feature in September, Bob Watcher, MD, chair of the department of medicine at UC San Francisco, said "knowing minute-to-minute heart rate variations in intensive care unit patients is sometimes useful. Whether it is in healthy 30-year-olds is an unproven hypothesis and should be tested. Likely to be tons of false positives, which will drive anxiety and some overtreatment."

Apple said its app accurately detects atrial fibrillation 99 percent of the time when it gets a good reading. However, Dr. Yazdi is concerned that accuracy may be compromised in younger patients, for whom atrial fibrillation is rare.

To calculate the probability that a wearer would actually have atrial fibrillation when it is detected by the Apple Watch, Dr. Yazdi combined data on the baseline risk of having atrial fibrillation from the AnTicoagulation and Risk Factors in Atrial Fibrillation Study with data Apple has provided on its EKG app's sensitivity and specificity — 98 percent and 99.6 percent, respectively.

Dr. Yazdi's analysis found that in people under the age of 55, the Apple Watch correctly diagnoses atrial fibrillation 19.6 percent of the time. This compares to 76 percent accuracy among users between the ages of 60 and 64, 91 percent among those aged 70 to 74, and 96 percent for those older than 85.

"Here's the bottom line: For the vast majority of individuals under age 55 whose Apple Watches tell them they have atrial fibrillation, the odds are high that the watch is wrong. But it is more accurate for the aging population that is becoming a part of the wearable generation," Dr. Yazdi writes.

To read the complete op-ed, click here.

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