In Covid-19, PaO₂/FiO₂ ratio (PF) predicts mortality in patients with acute respiratory failure (ARF) treated with invasive or non-invasive ventilatory support. 
Aim of this study was to evaluate if PF measured at emergency room (ER) admission predicts mortality in patients with Covid-19 ARF breathing room air or supplemental oxygen. 
We enrolled 433 patients (215 male; 218 female) with Covid-19 pneumonia and ARF (PaO₂<60mmHg and PF<300). PF was measured at ER (T₀), at ward admission (T₁) and at ARF nadir (T₂). To define PF accuracy to predict mortality we used receiver-operating characteristic (ROC) curve.
At T₀, PF median value was 174 (109-257); 37.2% of patients were in room air, 62.8% were on supplemental oxygen. At T₁, PF median value was 152 (116-196); almost all patients received positive pressure support (35.2%) or high flow oxygen therapy (34.9%). At T₂, PF median value was 149 (108-198); 57.2% of patients were on positive pressure support. Mortality at 28-day was 24.5%. Considering 28-day mortality, area under the ROC curve (AUROC) values were 0.6244 at T₀ [Fig.1], 0.6186 at T₁ and 0.6813 at T₂.
PF at ER is a good predictor of 28-day mortality in patients with Covid-19 ARF breathing room air or supplemental oxygen. Of notice, this is the first large retrospective study demonstrating the prognostic role of PF in very early stage of Covid-19.