P/F Ratio Explained
How the PaO₂/FiO₂ ratio guides ARDS diagnosis and severity.
The P/F ratio (PaO₂ divided by FiO₂) is a quick, bedside measure of how well oxygen is getting from the lungs into the blood. It is essential for diagnosing and classifying the severity of Acute Respiratory Distress Syndrome (ARDS).
How to Calculate the P/F Ratio
P/F Ratio = PaO₂ (mmHg) ÷ FiO₂ (decimal)
For example, if a patient has a PaO₂ of 80 mmHg on 40% oxygen (FiO₂ = 0.40):
80 ÷ 0.40 = 200
ARDS Severity Classification (Berlin Criteria)
| P/F Ratio | ARDS Severity |
|---|---|
| 300+ | No ARDS / Mild impairment |
| 200–300 | Mild ARDS |
| 100–200 | Moderate ARDS |
| < 100 | Severe ARDS |
Using the P/F Ratio on Room Air
If a patient is not intubated, you can estimate FiO₂ from supplemental oxygen delivery (e.g., nasal cannula ≈ 24–44% depending on flow rate). Our calculator includes these approximations for quick estimation.
Limitations
The P/F ratio does not account for positive end-expiratory pressure (PEEP) or the underlying cause of hypoxemia. It should be interpreted alongside the full clinical picture, chest imaging, and hemodynamic status.
Calculate the P/F ratio with our P/F ratio calculator.
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