3. Results

3.1. Literature Search Results
For the case of MetHb and COVID-19, a total of 201 articles were initially retrieved from PubMed (n = 6) and Google Scholar (n = 196) using the literature search strategy mentioned above. Based on the inclusion and exclusion criteria, seven duplicates were removed and 187 articles were excluded after abstract review. After the full set of remaining articles was screened, seven reports were finally included in the present study [17,18,19,20,21,22,23]. The study selection process is shown in Figure 1a.
For the case of COHb and COVID-19, 73 articles were retrieved from PubMed (n = 1) and Google Scholar (n = 72), one duplicate was removed and 68 articles were excluded after abstract review, leading to three articles that were finally included in the present study [20,24,25]. The study selection process is shown in Figure 1b.

3.2. Characteristics of the Included Studies
From the seven studies included in this review for the case of MetHb and COVID-19, two were cross-sectional studies [19,21] and five case reports [17,18,20,22,23]. The case descriptions reported findings of one single patient [17,20,22,23] or three patients [18]. Five studies were published in journals [17,18,20,21,22] and two on the preprint server medRxiv (medrxiv.org) [19,23].
For the case of the three studies concerning COHb and COVID-19, two were cross sectional studies [24,25] and one a case report [20]. Two of the studies were published in journals [20,25] and one on the preprint server Research Square (researchsquare.com) [24].

3.3. Summary of the Studies: MetHb and COVID-19
Kuipers et al. reported the case of a 56-year-old Afro-Caribbean man with a medical history of type 2 diabetes presenting to the emergency department with myalgia, dry cough and a peripheral O2 saturation (SpO2) of 94%, but not fever. COVID-19 was confirmed by a positive SARS-CoV-2 PCR test result. Bilateral ground glass opacities of the chest were detected by CT. In the following days, an increasing need for O2 administration was noted, SpO2 dropped to 83% and mechanical ventilation was initiated as well as treatment with chloroquine. Twelve hours later, hemolysis and an abnormally high level of MetHb (9.1%) were noted (Figure 2a). The patient was then treated with three units of packed red blood cells in the following 48 h and ascorbic acid (vitamin C) intravenously four times a day for two days, resulting in a normalization of MetHb levels within six days. A suspected glucose-6-phospate dehydrogenase (G6PD) deficiency was confirmed by genetic analysis.
Al-Aamri described a case of a 10–15-year-old Saudi girl (the exact age was apparently unknown) who was admitted to the hospital with a clinical presentation similar to Kawasaki disease shock syndrome, which she developed 22 days after a routine SARS-CoV-2 test that turned out to be positive. Her condition deteriorated in the following days while receiving multiple medications (Azithromycin, Favipiravir, Methylprednisolone, Enoxaparin, intravenous immunoglobulin, Aspirin, Tocilizumab, Norepinephrine, Epinephrine, Furosemide, Milrinon, insulin, blood transfusion, fresh frozen plasma, vitamin K and ascorbic acid), finally leading to her death at day 33. Intubation was performed and ventilation was started on day 29. The report mentioned that MetHb was 0.5–1.9% (min, max), implying that multiple MetHb measurements were made (Figure 2a). Exactly when the measurements were performed during the disease course was not mentioned. Genetic analysis confirmed a G6PD deficiency.
Palmer et al. reported the case of a 62-year-old Afro-Caribbean man with a medical history of type 2 diabetes and hypertension, presenting at the hospital after five days of fever, dyspnea, vomiting and diarrhea. A chest radiograph showed bilateral infiltrates, a kidney injury was diagnosed and a SARS-CoV-2 test returned positive. On admission, MetHb was 1.2%. The patient was treated with crystalloid fluid, O2 therapy, insulin, two blood transfusions, amoxicillin/clavulanic acid, heparin, amlodipine, metformin, and folic acid. The patient was discharged 22 days after admission. On day six, the highest MetHb value was noted (6.8%). A suspected G6PD deficiency was confirmed. The time-course of MetHb during the stay in the hospital is visualized in Figure 3d.
Faisal et al. [20] published a report on a case of a 74-year-old Afro-American man with a medical history of prostate cancer, hypertension and hyperlipidemia that presented to the clinic after seven days with fever, cough and progressively worsening shortness of breath. On admission, the person showed tachypnea, a SpO2 of 90% and returned a positive SARS-CoV-2 test result. Chest computed tomography (CT) revealed bilateral perihilar and right lower lobe opacities. After treatment with O2 therapy, azithromycin and hydroxychloroquine, his health worsened, and he was intubated and mechanically ventilated. He was then treated with lopinavir-ritonavir, ribavirin, tocilizumab, antibiotics, thiamine, hydrocortisone, ascorbic acid and norepinephrine. On day 15, hypoxia was noted (SpO2: 80–90%), but arterial blood gas analysis showed an arterial Hb saturation (SaO2) of 100%. MetHb was increased (6.3%). Treatment continued with intravenous ascorbic acid, hydroxocobalamin, and intravenous methylene blue. MetHb raised to 15.9%. After further treatment with intravenous methylene blue and red blood cell transfusion MetHb declined to 2–4%. The patient slowly recovered and was discharged on day 31 after admission. No genetic testing of a G6PD deficiency was performed.
Naymagon et al. [18] reported three cases of COVID-19 patients with MetHb measurements. The first case was a 50-year-old SARS-CoV-2 positive man with no medical history presenting with acute hypoxic respiratory failure in the clinic. He received hydroxychloroquine, azithromycin and ceftriaxone, and mechanical ventilation. He showed persistently low SpO2 values despite being mechanically ventilated. MetHb values increased steadily, peaking at 10.6% on day six (Figure 2a). Treatments with methylene blue and ascorbic acid normalized MetHb and his clinical status improved. A genetic analysis of G6PC deficiency was not done. The second case was a 52-year-old SARS-CoV-2 positive morbidly obese man with diabetes mellitus, admitted to the hospital due to acute hypoxic respiratory failure. He received hydroxychloroquine, azithromycin, cefepime, vancomycin, and apixaban, and was mechanically ventilated. A persistently low SpO2 was recognized and testing for MetHb resulted in a value of 22% (Figure 2a). He received methylene blue and ascorbic acid, but his MetHb increased to >30%. After receiving red blood cell transfusion, MetHb decreased to 2.9%. No genetic analysis of G6PD deficiency was performed. The third case was a 54-year-old SARS-CoV-2 positive man with diabetes mellitus admitted to the hospital due to acute hypoxic respiratory failure. He received azithromycin and hydroxychloroquine and was mechanically ventilated. He demonstrated persistent hypoxia and a MetHb of 13.7%. After receiving methylene blue, MetHb did not improve and increased to 18.8%. Shortly after, the patient died. Genetic testing revealed G6PD deficiency.
Alamdari et al. [21] investigated MetHb levels in 25 healthy individuals and 25 COVID-19 patients from Iran. Subjects with G6PD deficiency were excluded from the study. Patients showed a statistically significantly higher MetHb concentration in their blood compared to healthy controls (16.4 ± 9.1% vs. 2.5 ± 0.9%). According to the five cases reported in detail in this publication, the medical standard treatment included azithromycin and hydroxychloroquine. To treat the elevated MetHb, methylene blue, ascorbic acid and N-acetyl cysteine were administered. The authors concluded that it is crucial to treat the elevated MetHb in critically ill COVID-19 patients. Soltan et al. [19] used an artificial intelligence method and data from 115,394 emergency presentations and 72,310 admissions to a large UK teaching hospital group to predict COVID-19 cases. The data used were routinely collected data typically available within one hour during emergency presentations and admissions to hospital. Data from COVID-19 patients (n = 534) and pre-pandemic controls (n = 114,957) were included in the final analysis. Interestingly, while MetHb was a relevant parameter to be included in the models (see Figure 3a,b), MetHb was similar in the COVID-19 cohort compared to pre-pandemic controls (0.62% (0.4–0.8%) vs. 0.88% (0.6–1.1%)).
Figure 2a visualizes the summarized MetHb values reported by the seven publications.

3.4. Summary of the Studies: COHb and COVID-19
In their case report on a 74-year-old Afro-American man with COVID-19 already detailed in Section 3.2, Faisal et al. [20] described that, when the patient showed hypoxia and a MetHb concentration of 6.3%, COHb was 3.2%, i.e., above the normal reference range. After the patient was treated with intravenous ascorbic acid, hydroxocobalamin and methylene blue, COHb normalized (value not given in the report), while MetHb continued to rise.
Pawlowski et al. [25] found that, when comparing 246 SARS-CoV-2 PCR-positive patients to propensity-matched 2460 SARS-CoV-2 PCR-negative patients, COHb at clinical presentation was actually slightly, but significantly, higher in the SARS-CoV-2 PCR-negative population (0.99 vs. 0.57%) (Figure 2b). The same trend was seen until seven to nine days after admission.
Paccaudi et al. [24] analyzed the data of 63 patients admitted in the hospital for severe COVID-19 and found that, while COHb was not different for survivors in comparison to non-survivors (1.10 ± 0.50% vs. 0.95 ± 0.24%) (Figure 2b), COHb increased to statistically significantly higher values during the course of the hospital stay in non-survivors compared to survivors (see Figure 3c), leading the authors to conclude that a greater increase in COHb over time seems to represent a relevant marker of COVID-19 severity and seems to play a role in the determination of the survival probability of a COVID-19 infection.
Figure 2b visualizes the MetHb values reported by the seven publications summarized.