SpO₂/FiO₂ Ratio (S/F Ratio)
Assesses oxygenation using pulse oximetry and inspired oxygen fraction—useful when arterial blood gas is unavailable.
Calculator
How it works
What this tool does
It calculates the SpO₂/FiO₂ ratio (often written S/F), where both SpO₂ and FiO₂ are entered as percentages (e.g., SpO₂ 96% on 40% oxygen).
S/F is used as a non-invasive surrogate for the Horowitz index (PaO₂/FiO₂) in research and bedside triage; it is only one element of ARDS diagnosis and severity.
When it may help
- Acute respiratory failure when you want a quick oxygenation metric without an ABG.
- Trending oxygenation in ICU or ED when SpO₂ readings are reliable and FiO₂ is known.
- Not a substitute for full ARDS criteria (imaging, ventilation settings, PEEP, perfusion, altitude, dyshemoglobins, poor signal, etc.).
Formula
S/F = SpO₂(%) ÷ [FiO₂(%) ÷ 100] → equivalently S/F = 100 × SpO₂(%) ÷ FiO₂(%).
FiO₂ is the inspired fraction expressed as percent: 21% = room air, 40% = 0.40 fraction, 100% = pure oxygen.
References (selection)
- Rice TW, Wheeler AP, Bernard GR, et al. 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. (Describes correlation between S/F and P/F.)
- ARDS definitions (including Berlin and updated “global” ARDS criteria) incorporate PaO₂/FiO₂ or SpO₂-based thresholds with specific conditions (e.g., PEEP/CPAP or high-flow settings, SpO₂ ≤ 97% when using SpO₂). Always apply the full official definition.
Why use S/F?
- Less invasive and faster to obtain than repeated ABGs when trends matter.
- Published linear approximations link S/F to P/F for screening and research (example teaching points: S/F ≈ 315 corresponds roughly to P/F ≈ 300; S/F ≈ 235 to P/F ≈ 200 in adults in Rice et al.).
- Helps standardize communication about oxygenation across units that prefer pulse oximetry.
Limitations
- SpO₂ is inaccurate or unreliable in shock, severe anemia, carbon monoxide exposure, methemoglobinemia, poor waveform, motion artifact, and some nail pigments.
- Official ARDS SpO₂-based thresholds assume SpO₂ ≤ 97% in many definitions; high SpO₂ on high FiO₂ can mask severe shunt.
- Altitude, patient position, and device differences affect readings; do not use this ratio alone to change ventilator settings.
Dosing & care
Teaching interpretation (intubated ARDS oxygenation bands, S/F)
These bands mirror common SpO₂-based severity cutoffs used alongside full ARDS criteria—not a standalone diagnosis.
- S/F ≤ 148: corresponds to the “severe” oxygenation range in many intubated SpO₂-based schemes.
- S/F > 148 and ≤ 235: “moderate” range in many intubated schemes.
- S/F > 235 and ≤ 315: “mild” range in many intubated schemes.
- S/F > 315: above the usual “mild” SpO₂-based band; does not exclude illness if clinical context suggests hypoxemia by other means.
Non-intubated ARDS definitions may use SpO₂-based thresholds (often ≤ 315) with specific support conditions (e.g., high-flow or NIV/CPAP with minimum pressures/flows). See current guideline tables.
References (selection)
Adult correlation (Rice et al., 2007): S/F around 315 ≈ P/F around 300; S/F around 235 ≈ P/F around 200 (population averages, not patient-specific).
For education only. Not medical advice. ARDS diagnosis and severity require complete criteria, clinician judgment, and local protocols. Verify all calculations and device settings at the bedside.