Fig. 5

The ROC curve depicted an improvement in the discriminatory ability of the severity categories based on the APACHE II score (a) and PF ratio (b) with the addition of S1P level to the model. The AUC increased from 0.73 to 0.80 (P > 0.05) and 0.55 to 0.77 (P < 0.01), respectively. ROC receiver operating characteristic, APACHE II Acute Physiology and Chronic Health Evaluation II, PF PaO₂/FiO₂, S1P sphingosine-1-phosphate, AUC areas under the curve