Critical Care Equation Moderate evidence Clinical review pending v1.0.0

SpO₂/FiO₂ (S/F) Ratio

What is the non-invasive oxygenation ratio when no blood gas is available?

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About this tool

Pulse-oximetry based surrogate for the P/F ratio.

Intended use

Patients with SpO₂ ≤ 97% on known FiO₂.

Outcome

Oxygenation severity (surrogate)

Time horizon

Point in time

Limitations

Unreliable when SpO₂ > 97%.
Pulse oximetry bias in darker skin pigmentation.

Clinical problems

Respiratory failure and COPD

Evidence

Evidence level

Moderate evidence

Externally validated, or an established physiological equation whose clinical thresholds rest on consensus; guideline support limited or mixed. Methodology

Evidence summaries for this tool are awaiting independent clinical review. Statistics are shown only where they could be attributed to a specific publication.

References

Original publicationRice 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;132:410-417. Find on PubMed (opens in a new tab)

Version history

Version	Formula	Effective	Reviewer	Changes
v1.0.0	Rice 2007	2026-10-03	—	Initial implementation with reference test vectors.

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Clinical decision support only. Interpret results in the context of the individual patient and current guidance; HumanPulse does not replace professional medical judgment.