When your Kardia device detects atrial fibrillation, you’ll see two things: an irregular, jagged EKG tracing on screen and a text result that reads “Possible Atrial Fibrillation.” The waveform itself looks noticeably different from a normal reading, with an uneven rhythm and a messy, jittery baseline between heartbeats instead of the smooth, predictable pattern you’d see during normal sinus rhythm.
What a Normal Kardia Reading Looks Like
To spot AFib, it helps to know what normal looks like first. A healthy Kardia tracing has a repeating, rhythmic pattern. Each heartbeat cycle includes a small bump (called a P-wave), a tall spike (the QRS complex), and another gentle wave after it. The spacing between each tall spike is roughly even, like a metronome. When the recording finishes, the app labels it “Normal” and displays your heart rate.
That regularity is the key thing to watch. The tall spikes march across your screen at consistent intervals, and the space before each spike has that small, rounded bump showing your upper heart chambers firing in an organized way.
How AFib Looks Different on the Tracing
An AFib recording on Kardia breaks both of those patterns. The most visible change is the spacing between the tall spikes. Instead of landing at even intervals, they’re scattered. Some come close together, others are farther apart, with no predictable rhythm. If you tried to tap your finger along with them, you couldn’t keep a steady beat.
The second hallmark is what happens to the baseline between those tall spikes. In a normal reading, the line is relatively flat with that small, clean P-wave bump before each heartbeat. In AFib, the P-wave disappears entirely. Instead, the baseline becomes wavy, jittery, or finely irregular. This happens because the upper chambers of the heart are firing chaotically, hundreds of times per minute, rather than producing a single organized contraction. On a small phone screen, this can look like a slightly trembling or fuzzy line between beats rather than the dramatic zigzag you might expect.
The fibrillatory waves that replace the P-wave vary from person to person. Some people produce coarser, more visible waves. Others have finer, subtler ones that are harder to distinguish from a noisy signal on a consumer device. The dominant frequency of these chaotic electrical signals falls in the range of 4 to 9 cycles per second, which is fast enough to create that characteristic shimmering baseline.
What the App Tells You in Words
After your 30-second recording, the Kardia algorithm analyzes the tracing and assigns one of several labels. If AFib is detected, the app immediately displays “Possible Atrial Fibrillation.” Other possible results include “Normal,” “Bradycardia” (slow heart rate), and “Tachycardia” (fast heart rate).
There’s also a fifth result that catches many users off guard: “Unclassified.” This appears when the algorithm doesn’t detect AFib but also can’t confidently call the recording normal, fast, or slow. An unclassified result can be caused by other rhythm irregularities, heart rates that fall outside the algorithm’s expected range, or a recording with too much electrical noise. It doesn’t mean you have AFib, but it also doesn’t rule it out.
Artifacts That Mimic AFib
One of the most common frustrations with Kardia is getting a messy tracing that looks abnormal but is actually just noise. Muscle artifact is a frequent culprit. If your hands are tense, cold, or trembling while touching the electrodes, the tiny electrical signals from your finger and hand muscles bleed into the recording. This creates a jittery, irregular baseline that can look surprisingly similar to the fibrillatory waves seen in real AFib.
Baseline wander is another issue. If you shift your fingers on the electrodes or move during the recording, the entire tracing drifts up and down in slow waves, distorting the shape of each heartbeat. Electrical interference from nearby electronics can also introduce high-frequency noise that clutters the signal. All of these artifacts are significantly worse when you’re moving compared to sitting still and relaxed.
To get a clean recording, rest your arms on a table, relax your hands completely, and stay still for the full 30 seconds. Dry skin or cold fingers can also weaken the signal, so warming your hands and lightly moistening your fingertips sometimes helps.
How Accurate Is the AFib Detection
A study of adults aged 65 and older published in the Netherlands Heart Journal found that when KardiaMobile produced a clear, classifiable result, it detected AFib with 90.7% sensitivity and 98.6% specificity. That means it correctly identified about 9 out of 10 people who actually had AFib, and it correctly cleared about 99 out of 100 people who didn’t.
The catch is that the device produces a fair number of unclassified readings, especially in older adults. When those unclassified results were counted as “no AFib detected,” sensitivity dropped to 70.9%, meaning roughly 3 out of 10 AFib episodes were missed. Specificity stayed high at 98.8%. The practical takeaway: a “Possible Atrial Fibrillation” result is a strong signal worth acting on, but a normal or unclassified result doesn’t guarantee your rhythm is fine, particularly if you’re having symptoms.
What to Do With Your Recording
The most useful thing about a Kardia recording is that it captures your heart rhythm at the exact moment you feel something off. You can download any recording as a PDF directly from the app. Open the History tab, find the recording you want, tap the three dots in the upper right corner, and select “Download PDF.” The app generates a diagnostic-quality EKG document you can save to your phone or share via email or text.
This matters because AFib often comes and goes. By the time you get to a clinic, your heart may be back in normal rhythm, making it invisible on a standard EKG. Having a PDF of the episode in hand gives your doctor something concrete to evaluate. A consensus statement from the AF-SCREEN International Collaboration, published in the American Heart Association’s journal Circulation, emphasized that any consumer ECG recording suggesting AFib always requires confirmation by a healthcare professional experienced in reading EKGs. The device is a screening tool, not a diagnosis. But it’s a screening tool that captures evidence you can bring to your appointment, which is exactly what makes it valuable.