SpO2/FiO2 Ratio (SF Ratio)

Calculates the SpO2/FiO2 ratio as a non-invasive alternative to PaO2/FiO2 (PF) ratio when ABG is unavailable. Used for ARDS screening and severity classification, particularly in resource-limited settings.

%
Pulse oximetry oxygen saturation.
decimal fraction (0.21-1.0)
Fraction of inspired oxygen.

Enter SpO2 and FiO2 to calculate SF ratio

References

  1. Using SpO2/FiO2 Ratio to Classify ARDS in ICU Patients — European Medical Journal (2025)
  2. The Berlin definition of acute respiratory distress syndrome — European Respiratory Journal / PMC (2020)

Clinical Notes & Warnings

  • SF ratio correlates with PF ratio but is NOT identical. Cutoffs are approximate.
  • Published SF cutoffs vary: severe ARDS SF <=123 (one study), or SF <=146 (other studies).
  • At high FiO2 and SpO2 >97%, SF ratio loses discriminatory power (saturation ceiling effect).
  • Poor peripheral perfusion, dark skin pigmentation, and motion artifact can affect SpO2 accuracy.
  • ARDS diagnosis requires bilateral infiltrates, acute onset, and non-cardiogenic etiology - NOT just oxygenation criteria.
  • PEEP must be >=5 cmH2O for ARDS classification per Berlin definition.

SF ratio is particularly useful when ABG is unavailable or in resource-limited settings. Studies show SF ratio of ~123 corresponds to PF ratio of 100-110, and SF ~235 corresponds to PF ratio of ~200. At high saturation (>97%), the relationship becomes non-linear.