What’s a P/F Ratio and Why Should You Care?

The P/F ratio is a quick calculation you can do that will help you and the medical team determine if the patient has ARDS (acute respiratory distress syndrome).

First, find your PaO2 from your blood gas and the FIO2 on your ventilator (or from whatever oxygen delivery device you are using). For this example, let’s say your PaO2 is 83 and your ventilator 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.

Anything less than 300 is indicative of ARDS…but what about levels of severity? Glad you asked!

• 300 to 200 is considered mild ARDS
• 200 to 100 is considered moderate ARDS
• Anything below 100 is considered severe ARDS.

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 really be feeling confident in the ICU. You’ve got this!

Review the P/F ratio in episode 33 here.

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Last Updated on March 3, 2025 by Maureen Osuna, MSN, RN