APPS (age, plateau, PaO2/FiO2) score: a 9-point outcome score for patients with the acute respiratory distress syndrome. The score is based on patient’s age, PaO2/FiO2, and plateau pressure at 24 hours after ARDS onset. PaO2/FiO2 is calculated under standardized ventilator settings [see reference (5) for details]. Total score is equal to the sum of the points for each category at 24 hours after ARDS diagnosis. Minimum total score is 3 points; maximum score is 9 points.