Self-monitoring of fat metabolic status with smartphone-assisted breath acetone detector

We present a handheld breath acetone detector for metabolic self-monitoring at home. The device enables volume-controlled, end-tidal breath sampling through visual guidance, ensuring reproducible and accurate quantification of breath acetone. Small inter-device variability was identified (±5.6% for 3 devices at 1 part per million [ppm]), with humidity robustness (≤4.2% bias across 0%–90% relative humidity) and stability over 8 months (6.1% inter-day variability). We tested the detector in 12 volunteers, including subgroups undergoing exercise and dietary interventions, to evaluate its performance under realistic conditions, capturing rapid subject-specific changes in breath acetone levels, in agreement with blood ketone and interstitial glucose assessments. Validation against proton transfer reaction time-of-flight mass spectrometry is demonstrated across a physiologically relevant range of 0.2–45 ppm (e.g., 7.5% bias at 2 ppm) in 312 breath samples.