What's the difference between passive AFib detection on my watch versus the ECG strip function?

These are meaningfully different diagnostic tools. Passive PPG-based rhythm detection runs continuously in the background: the watch analyzes your heart rate pattern for irregularity consistent with atrial fibrillation. This method achieves approximately 98% specificity (very few false positives) but only about 67% sensitivity (missing roughly one in three AFib cases). It is better at ruling AFib in than ruling it out.

The single-lead ECG function requires active engagement: you press your finger on the digital crown for 30 seconds, the watch completes an electrical circuit, and generates an actual ECG trace equivalent to lead I. This method has an 84% positive predictive value for AFib when it flags a notification. More importantly, it generates a visual trace that I can actually look at and assess, the difference between probable AFib, premature atrial contractions (PACs), and motion artifact is often visible in the waveform. Save every ECG trace your Apple Watch generates by going into Health app > Heart > ECG. Screenshot it. That trace is a clinical document, not just a notification. (Perez et al., JACC, 2019)

Cardiologist's calibrated position, Promising (2) for passive PPG AFib detection. Promising (2) for single-lead ECG, which is more specific and produces concrete visual data.

What to do: After any irregular rhythm notification, immediately take an active ECG reading using the digital crown function. This gives your cardiologist something to read rather than just a passive alert to interpret.

For the full picture, read What Your Apple Watch Is Trying to Tell You.