<qa>  What is the most appropriate tool for stratifying lung severity in patients with ARDS?The PaO2FiO2 can be easily manipulated. Alterations in PEEP and FiO2 can dramatically change the PaO2FiO2. Despite recent reports on the effects of standardized ventilator settings on PaO2FiO2, is still a matter of debate whether the assessment of hypoxemia must be performed at 24 hours instead of at ARDS onset, and whether the assessment of hypoxemia under standardized ventilator settings is the most appropriate tool for stratifying lung severity in patients with ARDS. 