Clinical Tool
P/F Ratio
ARDS classification · Oxygenation index
P/F Ratio
381
Berlin Definition (2012)
| Severity | P/F Ratio | Mortality |
|---|---|---|
| Normal | ≥400 | — |
| Mild | 200–300 | 27% |
| Moderate | 100–200 | 32% |
| Severe | <100 | 45% |
About the P/F ratio calculator
The P/F ratio is the arterial oxygen tension divided by the inspired oxygen fraction. It is the standard way of expressing how much oxygenation a patient is achieving for the oxygen being given, and it is the measurement the Berlin definition of ARDS is built on.
The Berlin severity bands
Under the Berlin definition, ARDS is graded as mild at a P/F of 200–300, moderate at 100–200, and severe at 100 or below. The bands are only meaningful when the patient is receiving at least 5 cmH₂O of PEEP or CPAP, because PEEP itself changes the ratio.
The definition also requires that the findings are not fully explained by cardiac failure or fluid overload, and that there are bilateral opacities with an onset within a week of a known insult. A P/F ratio on its own is a number, not a diagnosis.
Why the same lung gives different ratios
The ratio moves with FiO₂ even when the lung has not changed, and the effect is largest at high FiO₂ and with a significant shunt. The same patient can shift a severity band on an FiO₂ change alone, which is one reason serial values matter more than any single one.
Altitude matters too. At elevation the barometric pressure is lower, the alveolar oxygen tension falls with it, and the ratio drops without any change in lung function.
Reading it alongside the rest
A falling P/F ratio with a rising PEEP requirement is a different clinical story from a falling ratio at a fixed PEEP, and the oxygenation index or the A-a gradient may describe the change better. The ratio is most useful as a trend on stable settings.
Common questions
- What is a normal P/F ratio?
- A healthy lung is usually above 400. Below 300 is abnormal, and below 300 with the other Berlin criteria met is the threshold for ARDS.
- How do you calculate the P/F ratio?
- Divide PaO₂ in mmHg by FiO₂ as a decimal. A PaO₂ of 80 on 50% oxygen is 80 ÷ 0.5 = 160.
- Why does the P/F ratio need at least 5 of PEEP?
- PEEP recruits alveoli and improves oxygenation, so the same lung produces a better ratio on more PEEP. The Berlin definition fixes a minimum so that severity grades are comparable between patients.
- Is a P/F ratio of 150 severe ARDS?
- It falls in the moderate band, which is 100–200. Severe is 100 or below.
Related
Educational reference for respiratory therapy students and clinicians preparing for the NBRC examinations. It supports clinical reasoning and does not replace institutional protocol, a device's instructions for use, or the judgement of the team caring for the patient.