In pulmonary medicine, critical care and anesthesiology, the P:F ratio is the ratio of arterial oxygen concentration to the fraction of inspired oxygen. It reflects how well the lungs absorb oxygen from expired air. 
An example in a healthy person:
A higher ratio indicates better gas exchange:
- Normal is 500
- ARDS is < 200
- Caroline Helwick, P/F Ratio May Be a Marker of Potential Intraoperative Ventilator-Induced Lung Injury, "Coverage of the American Society of Anesthesiologists (ASA) 2009 Annual Meeting", Medscape Medical News
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