The P/F ratio (PaO₂/FiO₂) is a key marker of oxygenation and is used to classify ARDS severity per the Berlin Definition. Normal P/F ratio is >300. ARDS is diagnosed when P/F <300 with PEEP/CPAP ≥5 cmH₂O, bilateral infiltrates, and non-cardiac cause.
The Berlin Definition (2012) requires all three: (1) P/F <300 with PEEP ≥5, (2) bilateral opacities on imaging, (3) respiratory failure not fully explained by cardiac failure or fluid overload.
S/F ratio can estimate P/F when ABG is unavailable: S/F ≤315 corresponds to P/F ≤300 (mild ARDS threshold); S/F ≤235 to P/F ≤200.
For educational use only. Not a substitute for clinical judgment.
