SpO₂/FiO₂ ratio (S/F)
- Category: Respiratory
- Version: v1.0.1
- Reviewed: Reviewed 6 Sep 2026
- Validation: Validation pending
S/F = SpO₂ (%) ÷ FiO₂ (fraction). It is computed only for SpO₂ 80–97%. Above 97% the dissociation curve plateaus and the ratio cannot discriminate, so a value outside that window is rejected rather than scored. The bands are the PALICC-2 (2023) strata for non-invasive-ventilation PARDS (full-facemask CPAP/BiPAP with PEEP ≥ 5 cmH₂O): ≤ 150 is severe, > 150 to ≤ 250 is mild/moderate (≤ 250 meets the NIV-PARDS oxygenation criterion), and > 250 is above the threshold.
The 2024 global adult ARDS definition cuts S/F at ≤ 315, so a value between 250 and 315 is below the adult threshold while above the paediatric one. Both frameworks are correct, so read the number against the one intended. FiO₂ estimation on nasal cannula or low-flow is unreliable. Prefer P/F when a gas is available, and prefer OI/OSI on invasive ventilation.
| Result | Interpretation |
|---|---|
| ≤ 150 | ≤ 150 — Corresponds to the severe category for non-invasive-ventilation PARDS in PALICC-2 (2023) (full-facemask CPAP/BiPAP with PEEP ≥ 5 cm H₂O; SpO₂ ≤ 97%). |
| >150 to 250 | > 150 to ≤ 250 — Meets the PALICC-2 (2023) non-invasive-ventilation PARDS oxygenation criterion (S/F ≤ 250) and corresponds to the mild/moderate category (S/F > 150). |
| > 250 | > 250 — Above the PALICC-2 non-invasive-ventilation PARDS oxygenation threshold (S/F ≤ 250). Interpret in the full clinical context. |
References
- Emeriaud G, López-Fernández YM, Iyer NP, et al; PALICC-2. Executive Summary of the Second International Guidelines for the Diagnosis and Management of Pediatric ARDS (PALICC-2). Pediatr Crit Care Med. 2023;24(2):143–168.PMID 36661420DOI 10.1097/PCC.0000000000003147
- Khemani RG, Thomas NJ, Venkatachalam V, et al; PALISI. Comparison of SpO2 to PaO2 based markers of lung disease severity for children with acute lung injury. Crit Care Med. 2012;40(4):1309–1316.PMID 22202709DOI 10.1097/CCM.0b013e31823bc61b
- Rice TW, Wheeler AP, Bernard GR, et al; NIH NHLBI ARDS Network. 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.PMID 17573487DOI 10.1378/chest.07-0617
Reproduction rights
Freely reproducible.Arithmetic ratio; PALICC-2 numeric thresholds and the Rice/Khemani regression coefficients are facts, not copyrightable expression (pf-sf.md IP status). No verbatim scale-item wording is reproduced.
Independent clinical validation: pending
Two independent clinical validators will be named here once review is complete.
- 2026-09-03v1.0.0Initial releaseInitial published text.
- 2026-09-06v1.0.1ClarificationAdded a one-line description for the catalogue card. No rule, threshold or reference changed.
Other respiratory scores
Important. For use by qualified health professionals as an informational and educational aid. It supports clinical judgment and does not replace it. Verify every result independently before making a clinical decision. This is not a medical device.