Apple and Stanford's atrial fibrillation study is the largest ever for AFib

In 2017 Apple teamed up with Stanford University to launch the Apple Heart Study, focused on seeing if the Apple Watch could accurately identify atrial fibrillation, a leading cause of strokes and heart failure, killing nearly 130,000 in the US each year. Now, nearly a year later, the two are letting the people in on how the study worked.

Stanford announced it amassed 400,000 participants, making it the largest atrial fibrillation study ever. Each participant was required to have an iPhone and either an Apple Watch Series 1, 2 or 3. Apple also leveraged its massive customer base to invite people who qualified to participate in the study.

Read this: Atrial fibrillation explained

An app on iPhone would then intermittently check the heart rate data for measurements of an irregular pulse. If it found "sufficient episodes of an irregular pulse" it would alert the participant and tell them to visit a doctor involved with the study.

The participants would then get sent an electrocardiography patch they would have to wear for a whole week to record the rhythms of their heart, which would then be matched up against the readings from the Apple Watch.

The goal of the study was to ultimately see how accurate the Apple Watch was at identifying atrial fibrillation. Researchers wanted to see if participants who received notifications ended up being found to have AFib with the ECG patch, whether they ended up getting medical attention after receiving a notification and to see how accurate the Apple Watch's irregular pulse detection was compared to the ECG patch.

Apple submitted a subset of the data to the FDA for regulatory clearance, data that said the Apple Watch Series 4 is 98% accurate in detecting AFib. Stanford researchers were aware of the data submitted to the FDA, but did not see the data beforehand. In September, the FDA announced it approved two separate apps to read pulse data. One was for the new Apple Watch Series 4, which has an ECG on the device, and one was for previous Apple Watches.

While Apple and Stanford's study is large, there are some caveats. In an interview with Wired, Seth Landefeld, the chair of department of medicine at the University of Alabama and member of the US Preventative Services Task Force, a volunteer panel of experts in disease prevention, said more questions need to be asked.

The biggest question is to find out whether people who get screened for atrial fibrillation have fewer strokes long term. Additionally, Mintu Turakhia, cardiologist and lead author on the study, tells Wired that AFib is an indicator for more than just strokes, and can also be associated with other conditions from shortness of breath to heart failure.

Thus, it seems like Apple and Stanford will need to perform additional follow-up studies to see how effective Apple Watch AFib alerts are at helping people live healthier lives in the long term.

There's also an inconsistency. Apple and Stanford's study is set to finalize results in early 2019, and Apple is prepping to release irregular heart rate notifications and ECG on the Series 4 by the end of 2018. It's possible Apple feels comfortable enough with the results of the study thus far to introduce the health features to a mass audience, but there's also the chance that it's not ready, which could lead to misdiagnosis and over treatment.

Luckily for Apple, it has one of the largest data pools in medical research to draw from. Hopefully it can extend the data pool for additional research to help make the Apple Watch an even better health tool.