What is P/F Ratio Meaning in Medical Terms & How to Calculate it?

Having a medical condition can bring on many questions – from the symptoms you’re experiencing to the countless tests and procedures from your doctor. One bit of terminology you may have heard about but not fully understand is a P/F ratio. Knowing what this means and how it relates to your health can be both helpful and empowering, so let’s take some time to explore what precisely this ratio measures, its potential implications for your health, and why doctors evaluate it as part of their overall patient care.

P/F Ratio Explanation

The PaO2/ FiOratio, or P/F ratio, is valuable for ARDS and respiratory failure assessment. This ratio is calculated by taking a patient’s PaO2 (partial pressure of oxygen in the arterial blood) and dividing it by their FiO2 (fractional inspired oxygen).

A P/F Ratio less than 300 indicates acute respiratory failure:

Many physicians may not be aware of the P/F ratio. However, it has a long history of being used to assess ARDS, also known as acute lung injury, effectively. If the P/F ratio is

P/F Ratio Equivalent to a pO2 on room air Condition
≥400 ≥80 Normal
<400 60-59 Hypoxemia
<300 50-59 Respiratory
<250 40-49 Severe Respiratory Failure
<200 <40 Critical Respiratory Failure


The result is presented as a whole number. An average P/F ratio should be greater than or equal to 300, with a higher number indicating better blood oxygenation. A lower number may indicate an emerging problem, such as a decreased lung function or the onset of acute respiratory distress syndrome (ARDS).

Note- A normal P/F Ratio is ≥ 400 and equivalent to a PaO2 ≥ 80 mmHg on room air.

Example of Calculation the P/F Ratio: PaO2 / FIO2

The P/F ratio indicates what the pO2 would be on room air:

  • P/F ratio < 300 is equivalent to a pO2 < 60 mm Hg on room air
  • P/F ratio < 250 is equivalent to a pO2 < 50 mm Hg on room air
  • P/F ratio < 200 is equivalent to a pO2 < 40 mm Hg on room air

Example: Suppose the pO2 is 90mmHg on 40% oxygen (FIO2 = .40). The P/F ratio = 90 divided by .40 = 225. In this case, the pO2 on room air would have been about 45 mmHg (which is significantly < 60 mmHg on room air).

The PF ratio should not be used to diagnose acute-on-chronic respiratory failure since many patients with chronic respiratory failure already have a P/F ratio <300 (pO2 <60 mmHG) in their baseline stable state. That’s the reason they are treated with chronic supplemental home oxygen.

How does the PF ratio calculator work?

The PaO2 to FiO2 ratio is necessary to assess a patient’s oxygenation status. To perform this calculation, you need the PaO2 values obtained from a blood gas test and the FiO2 values, which can be either 21% or higher if the patient receives oxygen-enriched air.

Let’s say our patient has a PaO2 of 95 mmHg (a nutritional value between 75-100 mmHg) and a FiO2 of 30% (most likely receiving oxygen).

The equation then becomes:

PaO₂ / FiO₂ = PF ratio.

95 mmHg / 30 % = 316.67 mmHg

This yields a result of 316.67mmHg, shows a healthy PF ratio of> 400 mmHg, and therefore requires a doctor’s consultation.

How to Calculate FIO2 from Liters of Oxygen

A nasal cannula provides supplemental oxygen of up to 6 liters per minute (LPM). The fractional inspired oxygen (FIO2) can be calculated using the following equation:

FIO2 = (LPM of O2 / 24) + 0.21

For example, if the patient receives 4 liters of oxygen per minute, the FIO2 is (4/24) + 0.21 = 0.33 or 33%.

How to Calculate SpO2 to PaO2 (when no ABG available)

PaO2 is the partial pressure of oxygen in arterial blood, measured with an arterial blood gas (ABG). If no ABG is available, the arterial partial pressure of oxygen (PaO2) can be estimated using the oxygen saturation (SpO2) and a fraction of inspired oxygen (FiO2):

PaO2 = (0.0031 * FiO2) + SpO2

For example, if SpO2 is 93% and FiO2 is 0.4 (40%), the estimated PaO2 would be (0.0031 * 0.4) + 93 = 93.87mmHg

*Note -the SpO2/PaO2 conversion becomes unreliable when SpO2 is > 98%, but PaO2 of 110 mmHg for 97% may be used as a substitute to avoid an overestimation.

Potential Implications 

A P/F ratio can help your doctor determine if you are at risk for developing ARDS. It can also monitor your progress if you are already being treated for ARDS. ARDS pf ratio will generally rise as treatment progresses, and a decrease in the ratio ARDS may indicate that your condition is worsening.

Why Doctors Evaluate This Ratio

The P/F ratio provides a more reliable number than arterial blood gas measurements alone and is an important marker that your doctor can use to assess the oxygenation of your body. It can provide your doctor with a more detailed understanding of the severity of your condition and help them determine the best course of action for your care.

Key Takeaway

By understanding the P/F ratio and its implications for your health, you can actively manage your condition and work with your doctor to ensure the best possible outcome. Working together, you can use this ratio as a tool that will provide insight into your condition, help you recognize any changes in oxygen levels, and allow you to take steps to prevent further complications.

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