What’s a P/F ratio and why should you care?

PF Ratio

The P/F ratio is a quick calculation you can do that will give you some clues as to whether your patient is A-OK pulmonary wise or heading toward ALI (acute lung injury) or ARDS (acute respiratory distress syndrome).

First, find your PaO2 from your blood gas and the FIO2 on your vent. For this example, let’s say your PaO2 is 83 and your vent is set at 45% FIO2.

Next, convert your FIO2 into a decimal…in this case it is 0.45

Then, just simply divide your PaO2 by the FIO2…it will look like this: 83/0.45 = 184.

A P/F ratio less than 300 is suggestive of ALI, while a P/F ratio less than 200 is suggestive of ARDS,  which is the most severe form of ALI and super scary. Of course, you never just go by numbers alone…always look at your patient and take in the whole picture (suddenness of onset, chest x-ray, etc….).

In another post, we’ll look at the alveolar-arterial gradient and then you’ll be feeling super smart as you chat with those awesome respiratory therapists and make recommendations to the MD. You got this!

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2 thoughts on “What’s a P/F ratio and why should you care?

  1. David W. Casas

    As a practicing Respiratory Therapist for over 40 years, I sincerely appreciate working with a Nurse who can use these various ABG sample analysis result diagnostic devices WITH a careful patient assessment. For example: It is important to use thorough patient assessment to distinguish fluid overload, cardiac failure, I/O status, and ARDS. Lasix has improved many ABG results. Thank you for your great web site. It is also nice to be thought of as “cute” at my advanced age.

    1. Nurse Mo Post author

      Thanks for your input, David! And yes…all RTs are cute simply because they make such meaningful contributions (and do great chest compressions!).


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