P/F Ratio Calculator (PaO₂/FiO₂)
Calculate the PaO₂/FiO₂ oxygenation ratio.
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P/F ratio
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FiO₂ used
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About
The P/F ratio divides arterial oxygen tension by the inspired oxygen fraction. It summarizes oxygenation at a particular PaO₂, FiO₂, ventilatory setting, and time. A ratio alone is insufficient to classify ARDS: timing, bilateral imaging findings, edema origin, and the applicable PEEP/CPAP or respiratory-support criteria also matter. FiO₂ from variable-performance oxygen devices may be an estimate.
Formula
Interpretation
Reference context: Ratios above 300, 200–300, 100–200, and ≤100 mmHg are commonly discussed in oxygenation literature, but are not stand-alone clinical categories.
Limitations: Record oxygen device, estimated or measured FiO₂, PEEP/CPAP, and the timing of the arterial sample. Changes in these conditions can change the ratio.
Worked example: PaO₂ 80 mmHg at FiO₂ 40% uses 0.40 and gives 80/0.40 = 200 mmHg.
FAQ
Disclaimer
Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.