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PaO₂/FiO₂ (P/F) Ratio with Berlin ARDS Severity

The PaO₂/FiO₂ ratio quantifies hypoxemia. Under the Berlin definition (with PEEP or CPAP ≥5 cmH₂O, acute onset, bilateral opacities and no cardiac explanation), ARDS is mild at 201–300 mmHg, moderate at 101–200 and severe at ≤100. Normal is roughly 400–500.

PaO₂ (arterial) (mmHg or kPa)
FiO₂ (% or fraction) (%)
PEEP / CPAP (cmH₂O)

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Interpretation

Result	Meaning
>300	Not ARDS range (normal ~400–500)
201–300	Mild ARDS (Berlin)
101–200	Moderate ARDS
≤100	Severe ARDS

Formula

P/F = PaO₂ (mmHg) ÷ FiO₂ (fraction). Berlin severity applies with PEEP or CPAP ≥5 cmH₂O.

Clinical pearls

Berlin ARDS also requires onset within 1 week, bilateral opacities not explained by effusion/collapse/nodules, and respiratory failure not fully explained by cardiac failure or fluid overload.
Use FiO₂ as a fraction (0.21–1.0); with kPa PaO₂, ARDS cut-offs are ~40, 26.7 and 13.3 kPa.
At altitude >1000 m the Berlin definition suggests correcting: P/F × (barometric pressure / 760).

Examples

pf ratio pao2 80 fio2 50% peep 8 P/F for PaO2 9 kPa on FiO2 0.4 with PEEP 10 pao2 fio2 ratio 250 on room air

FAQ

What P/F ratio is severe ARDS?

A PaO₂/FiO₂ of 100 mmHg or less on PEEP ≥5 cmH₂O defines severe ARDS under the Berlin definition.

What is a normal P/F ratio?

About 400–500 mmHg breathing room air at sea level.

Evidence

ARDS Definition Task Force; Ranieri VM et al. Acute respiratory distress syndrome: the Berlin Definition. JAMA 2012;307:2526–33.

Pulmonary & Critical Care · Reviewed 2026-10-14 · Methodology