Apple is working with partners to test whether its smartwatch can be used to detect common heart conditions, an effort that would make its device a "must have" for millions of people worldwide.

The company is partnering up with a group of clinicians at Stanford, as well as telemedicine vendor American Well, to test whether Apple Watch's heart rate sensor can detect abnormal heart rhythms in a cohort of patients, according to two people familiar. The people requested anonymity as these plans have not yet been made public.

Arrhythmias, or abnormal heart rhythms, aren't always problematic. But in some people, a condition known as atrial fibrillation can show no external symptoms while carrying a risk of blood clots, strokes and other complications.

For that reason, an Apple Watch could be a useful screening tool for high-risk patients -- if its heart rate monitor proves to be sufficiently sensitive and accurate.

"Atrial fibrillation is a common rhythm disorder and knowing someone has it is medically useful because those people might need specific treatments," said Bob Wachter, chair of the Department of Medicine at the University of California San Francisco.

American Well declined to comment on its involvement with Apple. But the company's CEO Roy Schoenberg did say that telemedicine companies are working closely with wearable makers. If a problem is detected, he said, "the best route forward is to put a health care professional out in front."

Companies like American Well already provide apps for the iPhone so that anyone with an Internet connection can reach a doctor in a matter of minutes.

Apple Watch has also been used in studies to screen for heart rhythm abnormalities. A start-up called Cardiogram released the results of its research in May, in partnership with clinicians at UCSF.

Greg Marcus, a cardiac electrophysiologist at UCSF who was involved with the Cardiogram study, said Apple benefits from the real-time access to raw data from its heart rate sensor. "That's potentially more powerful," he said, than the signals that third-party developers can access through Apple Watch.

The clinical study is slated to kick off later this year, one of the people said.

Apple's Tim Cook hinted at the company's interest in heart health applications in an interview with Fortune published on Monday.

"We started working on the Apple Watch several years ago," he said, and one goal was "performing some measurements of your health that people were not measuring, at least continually. Like your heart. Very few people wore heart monitors. We're extremely interested in this area. And yes it is a business opportunity."

Cook went on to describe the medical health activity market as the "largest or second largest component" of the economy.

In June, CNBC reported that Apple hired Sumbul Desai, a rising star on Stanford's digital health team who was working on projects related to Apple Watch. CNBC also reported that month that Apple has been in talks with developers, hospitals and other industry groups about bringing clinical data, such as detailed lab results and allergy lists, to its devices.

Apple declined to comment. Stanford did not return a request for comment.