SpO₂/FiO₂ Ratio
Assesses lung function, particularly in patients with acute respiratory failure.
Advice
Interpreting results for adults (Rice, 2007):
- An S/F ratio of 315 approximates a P/F ratio of 300.
- An S/F ratio of 235 correlates with a P/F ratio of 200.
For use in ARDS:
S/F Ratio Thresholds for Specific ARDS Categories (per the “global definition” of ARDS)
| Category | S/F ratio criteria* |
| Nonintubated ARDS | ≤315 on HFNC with flow of ≥30 L/min or NIV/CPAP with ≥5 cm H2O end-expiratory pressure |
| Intubated ARDS |
Mild: >235 and ≤315 Moderate: >148 and ≤235 Severe: ≤148 |
| Modified definition for resource-limited settings | ≤315** |
*If SpO2 ≤97%
**Neither positive end-expiratory pressure nor a minimum flow rate of oxygen is required for diagnosis in resource-limited settings.
Keep in mind that the S/F ratio is just one of several criteria used in the diagnosis and severity classification of ARDS.