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PaO₂/FiO₂ (P/F) Ratio
Pulmonology / Critical Care / ARDS
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.
Arterial O₂ tension from ABG
If >1, interpreted as percentage
For S/F ratio calculation
Berlin Def. requires ≥5
Formula
P/F Ratio = PaO₂ (mmHg) / FiO₂ (fraction)
S/F Ratio = SpO₂ (%) / FiO₂ (%)
Berlin Definition requires PEEP/CPAP ≥5 cmH₂O
S/F 315 ≈ P/F 300 | S/F 235 ≈ P/F 200
S/F Ratio = SpO₂ (%) / FiO₂ (%)
Berlin Definition requires PEEP/CPAP ≥5 cmH₂O
S/F 315 ≈ P/F 300 | S/F 235 ≈ P/F 200
ARDS Berlin Classification
| P/F Ratio | Classification | ARDS Severity | Est. Mortality |
|---|---|---|---|
| >300 | Normal | — | Low |
| 200–300 | Mild | Mild ARDS | ~27% |
| 100–200 | Moderate | Moderate ARDS | ~32% |
| <100 | Severe | Severe ARDS | ~45% |
Clinical Notes
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.
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.
