How to Use PF Ratio Calculator
Enter arterial PaO2 in mmHg and FiO2 as a decimal. The PF Ratio Calculator divides oxygen pressure by delivered oxygen fraction and displays a broad category: normal or near-normal, mild, moderate, or severe abnormality.
FiO2 is the most common entry mistake. Type 0.40 for 40% oxygen, not 40. A PaO2 of 80 on FiO2 0.40 gives a PF ratio of 200.
The ratio is most useful when PaO2 and FiO2 are measured at the same time. Oxygen changes before or after the blood gas can make the ratio less meaningful.
Formula & Theory — PF Ratio Calculator
The PF Ratio Calculator uses the following formula or scoring rule:
PF ratio = PaO2 ÷ FiO2PF ratio normalizes arterial oxygen pressure by inspired oxygen concentration. This allows rough comparison between patients or time points with different oxygen settings.
Common teaching bands are greater than 300, 200-300, 100-200, and below 100. These bands are often discussed in acute lung injury and ARDS education.
The ratio does not include airway pressure, hemoglobin, cardiac output, altitude, or ventilator mode. Oxygenation index adds MAP when pressure support matters.
Use Cases for PF Ratio Calculator
The PF Ratio Calculator is useful in specific situations such as:
- quickly checking blood gas oxygenation severity
- teaching why FiO2 must be entered as a decimal
- comparing oxygenation before and after an FiO2 change
- contrasting PF ratio with oxygenation index