P:F ratio

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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. [1]

An example in a healthy person:

A higher ratio indicates better gas exchange:

  • Normal is 500
  • ARDS is < 200

Comparative studies suggest this measure correlates better with pulmonary shunts than does the A-a gradient.[2][3][4]


  1. 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
  2. Covelli HD, Nessan VJ, Tuttle WK (1983). "Oxygen derived variables in acute respiratory failure". Crit. Care Med. 11 (8): 646–9. PMID 6409506[e]
  3. El-Khatib MF, Jamaleddine GW (2004). "A new oxygenation index for reflecting intrapulmonary shunting in patients undergoing open-heart surgery". Chest 125 (2): 592–6. PMID 14769743[e]
  4. Cane RD, Shapiro BA, Templin R, Walther K (1988). "Unreliability of oxygen tension-based indices in reflecting intrapulmonary shunting in critically ill patients". Crit. Care Med. 16 (12): 1243–5. PMID 3191742[e]