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Background: There is an urgent need to rapidly distinguish Covid-19 from other respiratory conditions, including influenza, at first-presentation. Point-of-care tests not requiring laboratory-support will speed diagnosis and protect health-care staff. We studied the feasibility of using breath-analysis to distinguish these conditions with near-patient gas chromatography-ion mobility spectrometry (GC-IMS). Methods: Independent studies at Edinburgh, UK, and Dortmund, Germany, recruited adult patients with possible Covid-19 at hospital presentation. Participants gave a single breath-sample for volatile organic compounds analysis by GC-IMS. Covid-19 infection was identified by RT-qPCR of oral/nasal swabs together with clinical-review. Following correction for environmental contaminants, potential Covid-19 breath-biomarkers were identified by multi-variate analysis and comparison to GC-IMS databases. A Covid-19 breath-score based on the relative abundance of a panel of volatile organic compounds was proposed and tested against the cohort data. Findings: Ninety-eight patients were recruited, of whom 21/33 (63.6%) and 10/65 (15.4%) had Covid-19 in Edinburgh and Dortmund, respectively. Other diagnoses included asthma, COPD, bacterial pneumonia, and cardiac conditions. Multivariate analysis identified aldehydes (ethanal, octanal), ketones (acetone, butanone), and methanol that discriminated Covid-19 from other conditions. An unidentified-feature with significant predictive power for severity/death was isolated in Edinburgh, while heptanal was identified in Dortmund. Differentiation of patients with definite diagnosis (25 and 65) of Covid-19 from non-Covid-19 was possible with 80% and 81.5% accuracy in Edinburgh and Dortmund respectively (sensitivity/specificity 82.4%/75%; area-under-the-receiver-operator-characteristic [AUROC] 0.87 95% CI 0.67 to 1) and Dortmund (sensitivity/ specificity 90%/80%; AUROC 0.91 95% CI 0.87 to 1). Interpretation: These two studies independently indicate that patients with Covid-19 can be rapidly distinguished from patients with other conditions at first healthcare contact. The identity of the marker compounds is consistent with Covid-19 derangement of breath-biochemistry by ketosis, gastrointestinal effects, and inflammatory processes. Development and validation of this approach may allow rapid diagnosis of Covid-19 in the coming endemic flu seasons. Funding Statement: MR was supported by an NHS Research Scotland Career Researcher Clinician award. DMR was supported by the University of Edinburgh ref COV_29 Declaration of Interests: The authors have nothing to declare. Ethics Approval Statement: Two independent studies were set up in the emergency departments of the Royal Infirmary of Edinburgh (RIE, Institutional Review Board ref RIE, 20/SS/0042), UK, and Klinikum Dortmund (KD, Institutional Review Board ref IfADo 176/2020), Germany.