Although Apple, based on its internal research, claims that the Apple Watch (AW) ECG has a 98% sensitivity and a 99% specificity for detection of atrial fibrillation, doubts have been raised about its accuracy in the real world.
I have recently reported on Apple Watch’s inability to diagnose atrial fibrillation (AF) when the heart rate is >120 beats per minute. This inherent limitation means AW has a built-in reduced sensitivity (which was not present in the testing group.)
In a Research Letter published online Feb. 24th in Circulation, Dr. Marc Gillinov, reports on the accuracy of Apple Watch in a population of patients who were post cardiac surgery and therefore on cardiac telemetry with a high risk of going in and out of AF.
Rhythm assessments using the Apple Watch ECG were performed 3 times per day over 2 days on 50 patients. Comparison was made between the watch reading (Sinus rhythm, AF, or inconclusive) and an expert human interpretation of the PDF from the watch and simultaneously obtained telemetry rhythm strip.
The results were disappointing for the AW.
The AW4 notification correctly identified AF in 34 of 90 instances, yielding a sensitivity of 41%. Of 25 patients with at least 1 episode of AF, AF was identified in 19. Among patients in SR, none was designated as AF (ie, no false positives); however, rhythm was deemed inconclusive in 31% of patients, and there was no additional attempt to assess rhythm. Overall agreement between AW4 notification and telemetry was 61% (κ statistic = 0.33 [95% CI, 0.24–0.41]).
This confirms my prediction that AW would identify less than half of AF cases.
I have to believe that the 29 cases diagnosed as “inconclusive” were due to the AW AF inherent blinding limitation related to rapid heart rate. If we presume these would all have been correctly identified as AF (if the AW had not been hamstrung) then the sensitivity increases to 70%.
The authors of this article don’t seem to understand the difference between unreadable (meaning too much artifact to make a diagnosis) versus inconclusive (which Apple only uses when the AF is > 120 BPM.) They conclude by saying:
The unreadable (ie, inconclusive) rate reported in that study was 6% compared with 31% in this pilot study.
They have muddled together unreadable and inconclusive.
I do strongly agree with their final conclusions
Variations in sensitivity between these 2 studies suggest the need for further validation before this technology is adopted by the public for AF detection. Physicians should exercise caution before undertaking action based on electrocardiographic diagnoses generated by this wrist-worn monitor.
Indeed, any diagnosis from the Apple Watch itself should be confirmed by a cardiologist who is an expert at interpreting these single-lead ECG recordings.