Endogenous CO production is mainly due to the inducible heme oxygenase (HO-1) enzyme, which catalyzes the heme moiety of Hb to biliverdin and liberates CO during this process. CO can then react with Hb, leading to the formation of COHb. HO-1 is upregulated in case of oxidative stress and inflammation which leads to increased COHb production [73,74]. Hemolytic anemia facilitates the production process of COHb so that an increased COHb blood level can be seen as a manifestation of hemolytic anemia [75]. Since anemia and hemolysis possibly occur during the course of disease in COVID-19 patients [17,18,22,54], hemolytic anemia may also be responsible for their COHb elevation. Because intracellular NADPH depletion and consecutive oxidative stress with damaged erythrocytes (hemolysis) is typical for G6PD deficiency, it is not surprising that G6PD deficiency in COVID-19 patients seems to be associated with elevated MetHb and COHb levels [20].