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How the S/F ratio is calculated
The SpO2/FiO2 (S/F) ratio is a validated non-invasive surrogate for the PaO2/FiO2 (P/F) ratio, used to grade hypoxaemia severity — including ARDS — when an arterial blood gas isn't immediately available.
S/F ratio = SpO2 (%) ÷ FiO2 (as a fraction, e.g. 0.21 for room air)
Example: SpO2 90% on FiO2 40% (0.4) → S/F = 90 ÷ 0.4 = 225.
Typical FiO2 by oxygen delivery device
| Device | Flow | Approx. FiO2 |
|---|---|---|
| Room air | — | 21% |
| Nasal cannula | 2 L/min | ~28% |
| Nasal cannula | 4 L/min | ~36% |
| Simple face mask | 6 L/min | ~40% |
| Simple face mask | 10 L/min | ~50% |
| Non-rebreather mask | 15 L/min | ~90%+ |
These are commonly used approximations, not fixed values — actual delivered FiO2 varies with the patient's own respiratory rate and tidal volume. When a ventilator or precise blender is in use, use its set/displayed FiO2 instead.
Normal values & severity interpretation
| S/F ratio | Interpretation |
|---|---|
| > 315 | No significant hypoxaemic lung injury by S/F criteria |
| 236 – 315 | Mild — approx. equivalent to PaO2/FiO2 ≤ 300 |
| ≤ 235 | Moderate–severe — approx. equivalent to PaO2/FiO2 ≤ 200 |
Clinical use
- ARDS screening without an ABG: the Berlin ARDS definition uses PaO2/FiO2 thresholds (≤300 mild, ≤200 moderate, ≤100 severe); the S/F ratio lets these be approximated non-invasively, which matters in resource-limited settings or when repeated ABGs aren't practical (the "Kigali modification" formalises this approach).
- Trend, not just a single number: a falling S/F ratio over time is a useful early warning sign of worsening respiratory failure, independent of the exact correlation to P/F.
- Reliability limits: the SpO2-based correlation flattens and becomes less reliable at SpO2 > 97–98% (the oxyhaemoglobin dissociation curve's flat upper portion) and in patients with poor pulse-oximeter waveform quality — confirm with ABG when the result would change management.
References
- Rice TW, 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.
- ARDS Definition Task Force (Berlin Definition). Acute respiratory distress syndrome. JAMA. 2012;307:2526–2533.
- Riviello ED, et al. Hospital incidence and outcomes of the acute respiratory distress syndrome using the Kigali modification of the Berlin definition. Am J Respir Crit Care Med. 2016;193:52–59.