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.
