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IntroductionThe novel coronavirus SARS-CoV2 and the associated disease, Covid-19, continue to pose a global health threat. The CovidCalculatorUK is an open-source online tool (covidcalculatoruk.org) that estimates the probability that an individual patient, who presents to a UK hospital, will later test positive for SARS-CoV2. The objective is to aid cohorting decisions and minimise nosocomial transmission of SARS-CoV2.MethodsThis n = 500 prospective, observational, multicentre, validation study compared the CovidCalculatorUK’s estimated probability of Covid-19 with the first SARS-CoV2 oropharyngeal/nasopharyngeal swab result for individual patients admitted to hospital during the study period (01.04.20 − 18.05.20). A comparison with senior clinicians’ estimates of the probability of Covid-19 was also made.ResultsPatients who were prospectively grouped, by the CovidCalculatorUK, into 0-30% estimated probability, 30-60% and 60-100% estimated probability went on to have first swab SARS-CoV2 positive results in: 15.7%, 30.5% and 61.9% of cases, respectively. CovidCalculatorUK performance demonstrated an area under the curve of 0.76 (95% CI 0.71 – 0.81) (p < 0.001). Senior clinician stratification of the estimated probability of Covid-19 performed similarly to the CovidCalculatorUK.ConclusionThe CovidCalculatorUK provides a reasonably accurate estimate of the probability of an individual testing positive on their first SARS-CoV2 nasopharyngeal/oropharyngeal swab. The CovidCalculatorUK output performs similarly to a senior clinician’s estimate. Further evolution of the calculator may improve performance.

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