SpO₂/FiO₂ Ratio (S/F Ratio)
S/F ≈ SpO₂ (%) ÷ FiO₂ (fraction). Educational approximation when ABG is unavailable.
Calculator
How it works
What this tool does
The SpO₂/FiO₂ (S/F) ratio is a non-invasive surrogate for the PaO₂/FiO₂ ratio used in ARDS research and bedside screening when arterial blood gas is not immediately available.
This page uses the common teaching approximation S/F ≈ SpO₂ (%) ÷ FiO₂ (%). Results are for education only.
When clinicians use it
- Suspected acute respiratory failure or ARDS when ABG is delayed or unavailable.
- Trending oxygenation on supplemental oxygen or non-invasive ventilation.
- Research and quality contexts that report S/F instead of P/F.
Formula (teaching)
S/F ≈ SpO₂ (%) ÷ FiO₂ (%)
Some references convert FiO₂ to a fraction (e.g. 40% → 0.40); this calculator uses percentage inputs for both values as on many wards.
References
- ARDS Definition Task Force. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526-2533.
- Rice TW et al. Comparison of the SpO₂/FiO₂ ratio and PaO₂/FiO₂ ratio in patients with acute lung injury. Chest. 2007;132(2):410-417.
Why it matters
- P/F ratio defines ARDS severity in Berlin criteria when obtained on standardized ventilator settings.
- S/F correlates with P/F in many cohorts but is not interchangeable for formal diagnosis.
- Pulse oximetry can be inaccurate with poor perfusion, motion, or extreme saturations.
Limitations
- Does not replace ABG, ventilator mechanics, or clinical examination.
- Not validated for all populations (e.g. severe anemia, CO poisoning).
- Berlin ARDS criteria require P/F with specific PEEP thresholds—not S/F alone.
Interpretation
Berlin-style bands (S/F teaching)
Approximate severity bands using S/F (not a substitute for formal P/F on ABG):
- S/F ≤ 235: severe range (rough teaching analogue to P/F ≤ 100 mmHg).
- S/F 236–315: moderate range (teaching analogue to P/F 101–200).
- S/F 316–395: mild range (teaching analogue to P/F 201–300).
- S/F > 395: above mild ARDS oxygenation threshold in many teaching charts.
Non-intubated patients may not meet Berlin timing/PEEP requirements even when S/F is low—use full diagnostic criteria.
References
Rice TW et al. described S/F as a surrogate for P/F in acute lung injury cohorts; confirm with ABG when management decisions depend on oxygenation.
Educational tool only—not for diagnosis or treatment decisions.