SpO₂/FiO₂ 比值

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

1. ARDS Definition Task Force. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526-2533.
2. 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.