evidence: There might also be a biological explanation for the leftward shift of the curve in COVID-19. Liu et al. put forward hypotheses about direct viral interaction with the heme group of hemoglobin. According to this theory, heme serum levels are increasing in COVID-19 along with harmful iron ions (Fe3+) causing inflammation and cell death (ferroptosis). This leads to the production of large amounts of serum ferritin to bind these free irons in order to reduce tissue damage [37]. In conclusion, SpO2 should be interpreted in the light of the presence of hyperventilation (tachypnea, low PaCO2) and, if possible, PaO2 via arterial puncture. Measuring the alveolar to arterial oxygen (P(A-a)O2 gradient (150 mmHg - PaCO2/0.8 - PaO2 at sea level) and relating this value to age and supplemental oxygen (age/4 + 4 + 50 (FiO2–0.21) in mmHg) can be insightful. This can be performed rapidly on a smartphone app [38]. The P(A-a)O2 gradient is increased either by V/Q mismatch or by intrapulmonary shunting. Hypoxemia due to V/Q mismatch can be easily corrected by supplemental oxygen therapy whereas pulmonary shunts have a poor response to oxygen therapy [39]. Causes of hypoxemia in COVID-19 [...] Liu W, Hualan L. COVID-19:attacks the 1-Beta chain of hemoglobin and captures the porphyrin to inhibit human Heme metabolism. ChemRxiv. 2020. Epub ahead of print.