Statistical analysis
Statistical analyses were performed using IBM SPSS Statistics for Windows version 22.0 (IBM Corp., Armonk, NY). Between-group comparisons were performed using Pearson’s chi-square test for parametric data and Mann–Whitney U test for non-normally distributed numerical data. The groups’ demographic data and laboratory parameters were compared using independent-samples t test. Wilcoxon analysis was performed to compare repeated measures for laboratory parameters within groups. Pearson correlation analysis was used to evaluate correlation between COHb and CRP, ferritin, D-dimer level, lymphocyte count, and the ratio of arterial oxygen partial pressure to fractional inspired oxygen (PaO2/FiO2). A p-value < 0.05 was considered statistically significant.