Microdroplets Fail to Retain Exhaled Volatile Biomarkers within a Single Breath
Exhaled breath condensate (EBC) contains volatile metabolites and is promising for non-invasive disease diagnosis, but after decades of research spanning over 100 biomarkers and 10 diseases, no EBC-based test has reached clinical use. The measurement variability that can span orders of magnitude, far exceeding the clinically required 10%, has long been attributed to biological factors. Here, we reveal a fundamentally different origin: the collected microdroplets themselves fail to retain volatile biomarkers. By isolating volatile co-condensation and transient evaporation from biological interference, we show that EBC microdroplets smaller than 100 {\mu}m lose clinically significant volatile content within a single breath cycle. This challenges the implicit assumption underlying decades of EBC research, that condensate faithfully reflects airway lining fluid. We develop and validate a physics-based model that predicts this loss across disease-relevant biomarkers and establishes the conditions for reliable EBC sampling. This work reframes EBC variability as a solvable engineering problem rather than an inherent biological limitation.