PaO₂/FiO₂ (P/F) Ratio Calculator
PaO₂/FiO₂ (P/F) Ratio Calculator
Calculate the P/F ratio, the oxygenation criterion in the Berlin definition of ARDS severity.
PaO₂/FiO₂ (P/F) Ratio
ARDS severityPaO₂ 90 mmHg, FiO₂ 60%
Formula
- PaO₂
- arterial oxygen partial pressure, mmHg
- FiO₂
- fraction of inspired oxygen — enter the percentage; the calculator divides by 100
- on room air
- FiO₂ = 21%, so P/F reduces to roughly 4.76 × PaO₂
Worked example
PaO₂ 90 mmHg, FiO₂ 60%
FiO₂ as a fraction = 60 ÷ 100 = 0.6
90 ÷ 0.6 = 150 mmHg
150 falls in the moderate ARDS band (100 – 200)
Berlin definition of ARDS severity
| P/F ratio | Severity |
|---|---|
| 100 or less | Severe |
| 100 – 200 | Moderate |
| 200 – 300 | Mild |
| > 300 | Not ARDS by oxygenation |
What the ratio does and doesn't tell you
The P/F ratio is simply arterial oxygen divided by the fraction of oxygen the patient is breathing, which makes it comparable across patients on very different levels of supplemental oxygen — a PaO₂ of 90 on room air and a PaO₂ of 90 on 60% oxygen represent very different degrees of lung injury, and the ratio captures that difference directly.
The Berlin definition uses three P/F bands — mild, moderate and severe — but the ratio alone does not diagnose ARDS. The definition also requires a minimum PEEP or CPAP of 5 cmH₂O, since an unsupported airway can produce a low ratio through atelectasis alone; bilateral opacities on chest imaging; and respiratory failure not fully explained by cardiac failure or fluid overload, distinguishing ARDS from cardiogenic pulmonary oedema.
The ratio is also sensitive to barometric pressure. It falls as pressure falls, so at altitude the same degree of lung injury produces a lower — apparently worse — ratio; a correction multiplying by (local barometric pressure ÷ 760) is sometimes applied to compensate. Where an arterial line is unavailable, the SpO₂/FiO₂ (S/F) ratio is a validated non-invasive substitute, tracking the P/F ratio closely once SpO₂ is below about 97%, above which the relationship flattens and loses precision.
Frequently asked questions
What does the P/F ratio measure?
Arterial oxygen (PaO₂) divided by the fraction of inspired oxygen (FiO₂), giving an oxygenation index comparable across patients on different levels of supplemental oxygen.
Does a low P/F ratio mean the patient has ARDS?
Not by itself. The Berlin definition also requires PEEP or CPAP of at least 5 cmH₂O, bilateral opacities on imaging, and respiratory failure not fully explained by cardiac failure or fluid overload.
Does altitude affect the P/F ratio?
Yes. The ratio falls as barometric pressure falls, so at altitude it overstates the severity of lung injury. A correction multiplying by (barometric pressure ÷ 760) can be applied.
What is the S/F ratio, and can I use it instead?
The SpO₂/FiO₂ ratio substitutes pulse oximetry for an arterial sample and correlates well with the P/F ratio, particularly below an SpO₂ of about 97%, making it useful when arterial access is not available.
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References
- ARDS Definition Task Force. Acute respiratory distress syndrome: the Berlin definition. JAMA. 2012;307(23):2526–33.
- Rice TW 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):410–7.