Background The clinical presentation of COVID‐19 ranges from a mild, self‐limiting disease, to multiple organ failure and death. Most severe COVID‐19 cases present low lymphocytes counts and high leukocytes counts, and accumulated evidence suggests that in a subgroup of patients presenting severe COVID‐19, there may be a hyperinflammatory response driving a severe hypercytokinaemia which may be, at least in part, signalling the presence of an underlying endothelial dysfunction. In this context, available data suggest a prognostic role of neutrophil‐lymphocyte ratio (NLR) in various inflammatory diseases and oncological processes. Following this rationale, we hypothesized that NLR, as a marker of endothelial dysfunction, may be useful in identifying patients with a poor prognosis in hospitalized COVID‐19 cases. Design A retrospective observational study performed at Hospital Universitario HM Puerta del Sur, Madrid, Spain, which included 119 patients with COVID‐19 from 1 March to 31 March 2020. Patients were categorized according to WHO R&D Expert Group. Results Forty‐five (12.1%) patients experienced severe acute respiratory failure requiring respiratory support. Forty‐seven (12.6%) patients died. Those with worse outcomes were older (P = .002) and presented significantly higher NLR at admission (P = .001), greater increase in Peak NLR (P < .001) and higher increasing speed of NLR (P = .003) compared with follow‐up patients. In a multivariable logistic regression, age, cardiovascular disease and C‐reactive protein at admission and Peak NLR were significantly associated with death. Conclusions NLR is an easily measurable, available, cost‐effective and reliable parameter, which continuous monitoring could be useful for the diagnosis and treatment of COVID‐19.

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