Exhaled breath analysis offers a painless window into airway inflammation, yet its clinical adoption is limited by a shortage of robust biomarkers. Hydrogen sulfide ( \(\mathrm {H_2S}\) ) has been detected in biological fluids, but its diagnostic value across asthma severities is not well established. In this study, we integrated an electrochemical \(\mathrm {H_2S}\) sensor with a detection limit of 1 ppb into the MISTRAL platform. We evaluated exhaled \(\mathrm {H_2S}\) in a prospective cohort of 28 adult asthmatic patients. Mean exhaled \(\mathrm {H_2S}\) concentrations were significantly higher in individuals with mild-to-moderate asthma ( \(192.4 \pm 57.1\;\textrm{ppb}\) ) than in those with severe asthma ( \(149.3 \pm 70.7\;\textrm{ppb}\) ; one‑tailed t‑test \(t = 1.72\) , \(p = 0.04\) ). In early-stage disease, \(\mathrm {H_2S}\) levels negatively correlated with maximal expiratory flow MEF \(\vphantom{0}_{50}\) , with a similar trend for MEF \(\vphantom{0}_{75}\) , whereas in severe asthma, positive but not fully statistically significant correlations emerged with FEV \(\vphantom{0}_1\) /FVC ratio, and vital capacity VC. These findings reveal a biphasic pattern: elevated \(\mathrm {H_2S}\) may signify a compensatory antioxidant response that collapses as damage in the medium-to-peripheral airways progresses. The plug-and-play MISTRAL platform thus enables real-time, point-of-care assessment of exhaled \(\mathrm {H_2S}\) as a promising, and complementary biomarker to FeNO and blood eosinophil count, offering a rapid, non-invasive indicator of disease control, treatment responsiveness, and exacerbation risk.
