What is a normal P/F ratio and what values indicate respiratory failure?

A normal P/F ratio is ≥400 mmHg. Values of 300–400 suggest mild hypoxemia; 200–300 indicates mild ARDS; 100–200 indicates moderate ARDS; and <100 indicates severe ARDS. In non-ARDS hypoxemia, P/F ratio <300 generally warrants close monitoring and supplemental oxygen titration, while <200 usually necessitates escalation of respiratory support.

Does FiO2 need to be recorded precisely for P/F calculation?

Yes. FiO2 should be measured or estimated as accurately as possible. For patients on high-flow nasal cannula, FiO2 can be estimated from flow rate using published nomograms. For mechanically ventilated patients, the set FiO2 is used directly. Inaccurate FiO2 estimation — especially with low-flow devices — significantly affects the calculated P/F ratio and may misclassify ARDS severity.

P/F Ratio (PaO2/FiO2) Calculator

Calculate the PaO2/FiO2 ratio to quantify oxygenation impairment and classify ARDS severity according to the Berlin Definition. Normal P/F ratio is ≥400.

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Frequently Asked Questions

What is a normal P/F ratio and what values indicate respiratory failure?

Does FiO2 need to be recorded precisely for P/F calculation?