PFratio

PFratio, commonly written as the P/F ratio, is a clinical index used to assess the efficiency of pulmonary oxygen transfer. It is defined as PaO2 divided by FiO2, where PaO2 is the arterial partial pressure of oxygen in millimeters of mercury and FiO2 is the fraction of inspired oxygen (expressed as a decimal). For example, a PaO2 of 80 mmHg on FiO2 0.40 yields a P/F ratio of 200. The ratio is most informative when measured under standardized ventilator settings and known oxygen delivery conditions.

Usage: It is central to the assessment of hypoxemia and is a key criterion in diagnosing and staging acute respiratory distress syndrome (ARDS). In the Berlin definition, ARDS is categorized as mild (P/F 200–300), moderate (100–199), and severe (≤100) when measured with a PEEP of at least 5 cm H2O.

Limitations: The P/F ratio does not reflect oxygen content (CaO2) or delivery, and readings can be affected by FiO2 accuracy, ventilator settings, intrapulmonary shunt, diffusion impairment, and measurement timing. It should be interpreted alongside other clinical data and, when possible, with PaO2/FiO2 values obtained with standardized PEEP.

Related metrics: The SpO2/FiO2 (SF ratio) is a noninvasive surrogate that correlates with the P/F ratio in many patients. Researchers and clinicians also use the Oxygenation Index (OI) in some settings to gauge the severity of respiratory failure and the need for escalation of support.