Background: COVID-19 is a new infectious disease with severe disease course and high mortality in some groups. Blood tests on admission to the hospital can be useful for stratification of patients and timely correction. Our study investigated the clinical features of COVID-19 patients in Latvia and differences in blood tests in groups with different disease severity. Methods: The retrospective study included 100 patients hospitalized in Riga East Clinical University Hospital in Spring 2020. The severity of the disease course was classified by the presence of pneumonia and its combination with respiratory failure. We have assessed blood cells' count, hemoglobin, hematocrit, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), alanine aminotransferase, lactate dehydrogenase (LDH), troponin T, electrolytes, creatinine, glomerular filtration rate (GFR), D-dimer, prothrombin time, prothrombin index, oxygen saturation, and temperature on admission to the hospital. Results: Patients were from 18 to 99, 57% males. Comorbidities were found in 74% of patients. The mild, moderate, and severe groups included 35, 44, and 16 patients, respectively. In the severe group, the mortality rate was 50%. The progression to severe COVID-19 was associated positively with temperature, ESR, CRP, creatinine, LDH, and troponin T and negatively associated with oxygen saturation, eosinophils, and GFR on admission to the hospital. Conclusions: COVID-19 severity associates with lower renal function and a higher level of inflammation and tissue damage. Eosinophils, CRP, ESR, LDH, troponin T, creatinine, and GFR are blood indicators for monitoring patients' condition.