BACKGROUND: Abnormal laboratory findings are common in patients infected with 
severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The aim of this 
systematic review was to investigate the effect of the level of some laboratory 
factors (C-reactive protein (CRP), creatinine, leukocyte count, hemoglobin, and 
platelet count) on the severity and outcome of coronavirus disease 2019 
(COVID-19).
METHODS: We searched PubMed, Web of Science, Scopus, and Google Scholar. We 
collected the articles published before May 26, 2020. We gathered the laboratory 
factors in groups of patients with COVID-19, and studied the relation between 
level of these factors with severity and outcome of the disease.
RESULTS: Mean CRP level, creatinine, hemoglobin, and the leukocytes count in the 
critically ill patients were significantly higher than those of the other groups 
(non-critical patients); mean CRP = 54.81 mg/l, mean creatinine = 86.82 μmol/l, 
mean hemoglobin = 144.05 g/l, and mean leukocyte count = 7.41 × 109. The 
lymphocyte count was higher in patients with mild/moderate disease (mean: 
1.32 × 109) and in the invasive ventilation group (mean value of 0.72 × 109), 
but it was considerably lower than those of the other two groups. The results 
showed that the platelet count was higher in critically ill patients (mean value 
of 205.96 × 109). However, the amount was lower in the invasive ventilation 
group compared with the other groups (mean level = 185.67 × 109).
CONCLUSION: With increasing disease severity, the leukocyte count and the level 
of CRP increase significantly and the lymphocyte count decreases. There seems to 
be a significant relation between platelet level, hemoglobin, and creatinine 
level with severity of the disease. However, more studies are required to 
confirm this.