A simple tool to help ruling-out Covid-19 in the emergency department: derivation and validation of the LDH-CRP-Lymphocyte (LCL) score

After the outbreak of the Covid-19 pandemic, cases of SARS-CoV-2 infections may gradually decrease in the next months Given the reduced prevalence of the disease, Emergency Departments (ED) are starting to receive more and more non-Covid-19 patients Thus, a way to quickly discriminate ED patients with potential Covid-19 infection from non-Covid-19 patients is needed in order to keep potentially contagious patients isolated while awaiting second-level testing In this paper, we present the derivation and validation of a simple, practical, and cheap score that could be helpful to rule out Covid-19 among ED patients with suspicious symptoms (fever and/or dyspnoea) The LCL score was derived from a cohort of 335 patients coming to the ED of our hospital from March 16th to April 1st, 2020 It was then retrospectively validated in a similar cohort of 173 patients admitted to our ED during April The score is based on blood values of lactate dehydrogenase, C-reactive protein, and lymphocyte count The LCL score performed well both in the derivation and in the validation cohort, with an AUC respectively of 0 81 (95% CI: 0 77 - 0 86) and of 0 71 (95% CI: 0 63 - 0 78), given the difference in Covid-19 prevalence between the two cohorts (57% vs 41% respectively) An LCL score equal to 0 had a negative predictive value of 0 92 in the derivation cohort and of 0 81 in the validation cohort, with a negative likelihood ratio respectively of 0 08 and 0 36 for Covid-19 exclusion This score could, therefore, constitute a useful tool to help physicians manage patients in the ED