How to Calculate the PF Ratio
The PF ratio is calculated by dividing the partial pressure of arterial oxygen (PaO₂) in mmHg by the fraction of inspired oxygen (FiO₂) expressed as a decimal: PF ratio = PaO₂ (mmHg) ÷ FiO₂ (decimal). 1, 2
Required Components
To calculate the PF ratio, you need two specific values:
- PaO₂: Obtain from arterial blood gas (ABG) analysis, measured in mmHg 1
- FiO₂: Express as a decimal (e.g., 40% oxygen = 0.40, room air = 0.21,100% oxygen = 1.0) 1
Step-by-Step Calculation
Convert FiO₂ percentage to decimal form by dividing by 100 (e.g., 60% = 0.60) 1
Divide PaO₂ by FiO₂ using the formula: PaO₂ ÷ FiO₂ = PF ratio 1, 2
Example calculation: If PaO₂ = 80 mmHg and FiO₂ = 0.40, then PF ratio = 80 ÷ 0.40 = 200 mmHg 1, 2
Clinical Interpretation
The resulting value classifies oxygenation status and ARDS severity according to the Berlin definition:
- Normal: 400-500 mmHg indicates healthy gas exchange 2
- Mild ARDS: 201-300 mmHg (mortality ~17%) 1, 2
- Moderate ARDS: 101-200 mmHg (mortality ~41%) 1, 2
- Severe ARDS: ≤100 mmHg (mortality ~58%) 1, 2
- Acute lung injury: Values <300 mmHg indicate need for escalating interventions 2
Critical Pitfalls and Considerations
The PF ratio varies significantly with the FiO₂ level used for measurement, meaning the same patient can be classified differently depending on the oxygen concentration at the time of measurement 3. In one study, 30% of patients changed disease classification when FiO₂ was altered 3.
Always document the FiO₂ level at which the PF ratio was measured when quantifying therapeutic interventions or specifying diagnostic criteria 3. This is essential because with the same shunt fraction, the PF ratio decreases as FiO₂ increases 4.
Serial measurements provide better prognostic information than a single value—patients demonstrating improvement in PF ratio within the first 24 hours have significantly better outcomes 2. Reclassifying the PF ratio 24 hours after initial management offers superior prognostic value compared to the initial measurement alone 2.
For severe hypoxemia (PF ratio <150 mmHg), consider preoxygenation with non-invasive positive pressure ventilation before intubation 2. When PF ratio is <150 within 7 days of onset, or <70 for ≥3 hours, or <100 for ≥6 hours, evaluate for venovenous ECMO 1.