PF Ratio Calculator (PaO2/FiO2)

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Choose the unit your blood gas analyser reports for PaO2.
Partial pressure of oxygen in arterial blood, from an arterial blood gas (ABG). Normal range is 75-100 mmHg (10-13.3 kPa) on room air.
mmHg
Percentage of oxygen in the inspired air. Room air = 21%. A ventilated patient set to FiO2 0.60 = 60% here.
%
The Oxygenation Index adds mean airway pressure (MAP) into the calculation, making it more sensitive to ventilator support intensity.
P/F RatioBelow normal (no ARDS)
381mmHg

PaO2 divided by FiO2 (expressed as a fraction). Normal >=400 mmHg.

ARDS SeverityBelow normal (no ARDS)
Estimated Mortality<10%
381 mmHg
Severe ARDS<100Moderate ARDS100-200Mild ARDS200-300Below normal300-400Normal400+

P/F Ratio 381 mmHg: Below normal (no ARDS)

  • A P/F ratio of 381 mmHg is below normal (400 mmHg) but above the ARDS threshold of 300 mmHg. Consider the clinical context.
  • A rough check: for an FiO2 of 21%, PaO2 should be approximately 105 mmHg if the lungs are healthy. Your measured 80 mmHg is below that, confirming impaired gas exchange.
  • P/F ratio is a tool, not a diagnosis. Interpret it alongside clinical assessment, chest imaging, ABG trend, and PEEP level (Berlin Definition requires PEEP >= 5 cmH2O for ARDS classification).

Next stepOxygenation appears adequate. Continue monitoring and address any clinical changes promptly.

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