We identified and analyzed nine studies that reported information about MetHb and COHb values of COVID-19 patients. 86% (6/7) studies reported an increased MetHb concentration above the normal reference range, and one study did not find such an increase. Regarding COHb, 67% (2/3) studies reported an increase, and one found no elevated COHb. As highlighted by the observations of Palmer et al. [22] and Paccaud et al. [24], the time of MetHb and COHb measurement during the course of the disease is critical, since MetHb and COHb seem to generally increase in hospitalized COVID-19 patients. The type of medical treatment is also of importance since some drugs currently used to treat COVID-19 have a high oxidizing potential provoking methemoglobinemia and carboxyhemoglobinemia.