What it is
The SpO₂/FiO₂ (S/F) ratio is a fully non-invasive oxygenation index for pediatric ARDS: it uses pulse-oximeter saturation instead of an arterial PaO₂, so no blood gas is needed. PALICC-2 uses it to diagnose non-invasive PARDS in children on CPAP/BiPAP or high-flow nasal cannula, where an arterial line is often neither present nor warranted.
The method
S/F = SpO₂ (%) / (FiO₂ as a decimal)
In the runner, FiO₂ is entered as a percentage and converted internally. Lower S/F = worse oxygenation. PALICC-2 sets the non-invasive PARDS threshold at S/F ≤ 253 (≈ P/F 300) on CPAP/BiPAP ≥5 cmH₂O or HFNC.
S/F → P/F anchors (Khemani)
The pediatric S/F values map onto familiar P/F bands:
- S/F 264 ≈ P/F 300
- S/F 221 ≈ P/F 200
- S/F 148 ≈ P/F 100
So the tool reads: >264 below threshold, 222–264 mild-range, 149–221 moderate-range, ≤148 severe-range (≈ P/F ≤100).
When to use it
Use the S/F ratio when you need to grade oxygenation in a child who is not intubated — on HFNC or non-invasive ventilation — or when an arterial gas is simply unavailable. It enables rapid, repeatable bedside severity assessment and is the natural non-invasive surrogate when the P/F ratio cannot be measured.
Worked example
SpO₂ 92% on FiO₂ 50%:
- S/F = 92 / 0.50 = 184
That sits between the 148 and 221 anchors → moderate-range PARDS (≈ P/F 100–200).
Pitfalls and caveats
- Valid only with SpO₂ ≤97%. Above ~97% the saturation plateaus and the ratio stops tracking true oxygenation — titrate FiO₂ down first.
- Enter FiO₂ as a percentage in this tool; by hand, divide by the decimal (0.50, not 50).
- It is a non-invasive surrogate: on an intubated child, grade severity with the Oxygenation Index (OI) or OSI, which incorporate mean airway pressure, and confirm with an arterial gas where possible.
- Reliable only with a clean plethysmograph trace and normal haemoglobin species — carboxy-/methaemoglobinaemia or poor perfusion corrupt the SpO₂.
- The anchors derive from pediatric data; do not transplant adult S/F cut-points.
Run it: SpO₂/FiO₂ Ratio (S/F Ratio)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.