P:F ratio

${P/F\ ratio}=\left({\frac {PaO_{2}}{Fi0_{2}}}\right)\times 100$

An example in a healthy person:

${476}=\left({\frac {100\ mm\ Hg}{21\%}}\right)\times 100$

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.