Introduction Early diagnosis and identification of potential critical cases for timely treatment are crucial for COVID‐19 patients. The aim of this study was to analyze the diagnostic and prognostic implications of WBC and cell population data (CPD) abnormalities related to COVID‐19 at disease onset. Methods Baseline WBC counts and CPD data were analyzed in one hundred COVID‐19 patients presenting to emergency department and subsequently discharged (n = 49), admitted (n = 51) or deceased (n = 22), and in 47 healthy subjects. Results Lymphopenia and eosinopenia were observed in all COVID‐19 patients, with more intensity in the admitted and deceased groups, that also presented increased WBC and neutrophil counts. On CPD analysis, COVID‐19 was associated with increased volume of neutrophils, lymphocytes, and monocytes, whereas conductivity was decreased for neutrophils and increased for lymphocytes. The ROC curve analysis showed good performance for lymphocyte counts in predicting COVID‐19 diagnosis (AUC = 0.858), for neutrophil counts in predicting admission for COVID‐19 (AUC = 0.744) and for monocytes volume in predicting COVID‐19 diagnosis (AUC = 0.837). Conclusion WBC counts and CPD parameters at disease onset in COVID‐19 patients can improve diagnostic characterization and aid in the discrimination between severe and nonsevere presentations.

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