261,435,768 COVID-19 infections were detected worldwide, of them 5,207,634 deaths were registered. Identifying markers of the patient severity early in the course of the disease can facilitate the assessment of the risk of adverse outcome.The objective: To compare values of laboratory parameters and their changes during treatment of patients with a complicated course of COVID-19 infection.Subjects and Methods. 56 patients were included in the study, all of them were hospitalized to COVID Hospital of the Clinic of Bashkir State Medical University, Russian Ministry of Health, from September 30, 2021 to November 15, 2021, and their complicated course of the disease necessitated transfer to the intensive care unit (ICU). The laboratory evaluation included the following: a general blood and urine counts, blood chemistry including urea and creatinine, liver transaminases, and blood coagulogram (prothrombin time (PTT), prothrombin index (PTI), thrombin time, fibrinogen, and blood clotting time).Results. In the group of patients with a fatal outcome on the day of transfer to ICU, lymphocytopenia, eosinopenia, elevated values of creatinine, total bilirubin, transaminases, C-reactive protein, D-dimer, and ferritin were noted. Also on this day, microscopic hematuria, proteinuria and cylindruria were detected in the urine tests of most patients in this group during treatment.Conclusion. Critical deviations in the results of hematological and biochemical tests were revealed. Particular attention should be paid to such parameters as the level of erythrocytes, lymphocytes, eosinophils, glucose, urea, creatinine, total bilirubin, aspartate aminotransferase, alanine aminotransferase, creatine kinase, C-reactive protein, D-dimer, and ferritin.