SpO2/FiO2 Ratio (S/F)

Non-invasive proxy for P/F ratio. S/F 235 ≈ moderate ARDS; S/F 315 ≈ mild ARDS.

For educational and clinical reference. Not a substitute for medical judgment. See the medical disclaimer.
S/F Ratio
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References

  1. Rice TW, Wheeler AP, Bernard GR, et al. Comparison of the SpO2/FiO2 ratio and the PaO2/FiO2 ratio in patients with acute lung injury or ARDS. Chest. 2007;132(2):410-417.

How to use

  1. Enter SpO2 (%) and FiO2 (0.4 or 40).
  2. S/F ratio appears instantly.
  3. S/F 235 corresponds to P/F ~200; S/F 315 to P/F ~300.

Frequently asked questions

Formula?

S/F ratio = SpO2 / FiO2 (FiO2 as fraction).

How does S/F relate to P/F?

Rice 2007: S/F 235 ≈ P/F 200 (moderate ARDS); S/F 315 ≈ P/F 300 (mild ARDS). Useful when ABG is unavailable.

When is S/F not reliable?

When SpO2 > 97% (saturation curve plateaus), in significant carboxyhemoglobinemia or methemoglobinemia, or with poor pulse oximetry signal.

Pediatric use?

S/F ratio is widely used in pediatric ARDS (PARDS criteria, PALICC 2015).

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