PaO2/FiO2 Ratio

Oxygen masks

The PaO2/FiO2 ratio, also known as the P/F ratio, is a quick calculation often used to determine the severity of ARDS.

The P/F ratio is calculated by dividing the arterial oxygen pressure (PaO2) obtained from an arterial blood gas sample by the FiO2 the patient is receiving.

Severity of ARDSP/F ratioMortality


  • The P/F ratio is inferior to the A-a gradient because it does not determine the source of hypoxemia. For example, the P/F ratio does not differentiate hypoxemia due to alveolar hypoventilation from other causes of hypoxemia.
  • The P/F ratio also does not factor in barometric pressure as part of the equation.

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Respiratory Therapist