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SpO₂/FiO₂ ratio (S/F)

Measured at steady state, not during a transient desaturation. Valid for S/F only when 80–97%; above 97% the ratio is not interpretable.

Room air is 0.21. Accepts a fraction or a percentage.

Independent clinical validation: pending

Two independent clinical validators will be named here once review is complete.

Validator slot open 1
Validator slot open 2

How it is calculated

S/F ratio = SpO₂ (%) ÷ FiO₂ (fraction), where SpO₂ is the pulse-oximeter saturation as a percent and FiO₂ is the inspired-oxygen fraction (0.21–1.0); the single reported value is dimensionless. Interpretation follows the PALICC-2 (2023) non-invasive-ventilation PARDS strata: S/F ≤ 150 corresponds to the severe category, > 150 to ≤ 250 to the mild/moderate category (S/F ≤ 250 meets the NIV-PARDS oxygenation criterion), and > 250 is above that threshold. The ratio is computed only for SpO₂ 80–97%; values outside that cited validity window are rejected rather than scored, because above 97% the oxyhemoglobin dissociation curve plateaus and S/F loses discrimination.

Limitations and notes

S/F is interpretable only for SpO₂ 80–97% (enforced as input bounds; above 97% the dissociation curve plateaus and the ratio cannot discriminate severity — a value above 97% is rejected, not scored). PALICC-2 restricts NIV-PARDS grading to full-facemask CPAP/BiPAP with PEEP ≥ 5 cm H₂O; FiO₂ estimation on nasal-cannula/low-flow is unreliable and the ratio should not be trusted there. When an arterial gas is available, PaO₂-based P/F is preferred; on invasive ventilation, OI/OSI are preferred over S/F. The published P/F↔S/F conversion equations (Rice 2007; Khemani 2009/2012) are notes only and are not computed here. No [NEEDS SOURCE] items apply to this score's inputs — the SpO₂ 80–97% window and FiO₂ 0.21–1.0 bounds are all cited; the research note's [NEEDS SOURCE] flags concern PaO₂/PF outer bounds, which are not inputs to S/F.

Accepted input ranges

  • Pulse oximeter oxygen saturation (SpO₂) — 80–97 %
  • Fraction of inspired oxygen (FiO₂) — 0.21–1 fraction

References

  1. 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
  2. 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
  3. 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

Version and changelog

v1.0.0 · Respiratory

  • 2026-07-25 v1.0.0 — Initial release: direct S/F ratio with PALICC-2 NIV-PARDS bands and the SpO₂ ≤ 97% validity guard.

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.