The oxyhaemoglobin dissociation curve is generally left-shifted in COVID-19 patients at admission to hospital, and this is associated with lower mortality.

Lung damage caused by SARS-Cov-2 virus results in marked arterial hypoxia, accompanied in many cases by hypocapnia. The literature is inconclusive as to whether these conditions induce alteration of the affinity of haemoglobin for oxygen. We studied the oxyhaemoglobin dissociation curves (ODCs) of 517 patients hospitalized with coronavirus disease 2019 (COVID-19) for whom arterial blood gas analysis (BGA) was performed upon hospitalization (i.e., before treatment). With respect to a conventional normal p50 (pO 2 at 50% saturation of haemoglobin) of 27 mmHg, 76% had a lower standardized p50 (p50s) and 85% a lower in vivo p50 (p50i). In a 33-patient subgroup with follow-up BGAs after 3, 6, 9, 12, 15 and 18 days' treatment, p50s and p50i exhibited statistically significant differences between baseline values and values recorded at all these time points. The 30-day Kaplan-Meier survival curves of COVID-19 patients stratified by p50i level show a higher probability of survival among patients who at admission had p50 values below 27 mmHg (p = 0.012). Whether the observed alteration of the affinity of haemoglobin for oxygen in COVID-19 patients is a direct or indirect effect of the virus on haemoglobin is unknown.