PaO2/FiO2 Ratio

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 ARDS	P/F ratio	Mortality
Mild	200-300	27%
Moderate	100-200	32%
Severe	<100	45%

Limitations

• 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.