P/F Ratio Explained
The P/F ratio, also called the PaO₂/FiO₂ ratio, is a quick way to compare a patient’s arterial oxygen level to the amount of oxygen they are receiving. Respiratory therapy students use it to assess oxygenation severity, monitor trends, and recognize ARDS-related hypoxemia patterns.
What Is the P/F Ratio?
The P/F ratio compares oxygen in the blood to oxygen being delivered. It helps answer the question: Is the patient’s PaO₂ appropriate for the FiO₂ they are receiving?
The arterial oxygen pressure measured on an ABG.
The fraction of inspired oxygen delivered to the patient.
A normalized oxygenation comparison.
Two patients can have the same PaO₂ but very different oxygenation severity if they are on different FiO₂ levels.
P/F Ratio Formula
Example: PaO₂ 80 mmHg on FiO₂ 0.40
80 ÷ 0.40 = 200
FiO₂ must be entered as a decimal. Room air is 0.21, not 21.
How to Convert FiO₂
Before calculating the P/F ratio, convert the oxygen percentage to a decimal.
| Oxygen Percentage | Decimal FiO₂ | Use in Formula |
|---|---|---|
| 21% | 0.21 | Room air |
| 30% | 0.30 | Use 0.30 |
| 40% | 0.40 | Use 0.40 |
| 60% | 0.60 | Use 0.60 |
| 100% | 1.00 | Use 1.00 |
P/F Ratio Severity Ranges
A lower P/F ratio means worse oxygenation. In ARDS classification, severity is commonly grouped by P/F ratio ranges.
| P/F Ratio | Oxygenation Interpretation | Common ARDS Severity Language |
|---|---|---|
| Greater than 300 | Better oxygenation | Not in ARDS P/F range |
| 201–300 | Mild oxygenation impairment | Mild ARDS range when ARDS criteria are met |
| 101–200 | Moderate oxygenation impairment | Moderate ARDS range when ARDS criteria are met |
| 100 or less | Severe oxygenation impairment | Severe ARDS range when ARDS criteria are met |
Board reminder: The P/F ratio alone does not diagnose ARDS. ARDS also requires clinical context, imaging, timing, and exclusion of primarily cardiac causes.
Worked Examples
PaO₂ 95 on room air
95 ÷ 0.21 = 452
Better oxygenation
PaO₂ 80 on FiO₂ 0.40
80 ÷ 0.40 = 200
Moderate impairment range
PaO₂ 70 on FiO₂ 0.60
70 ÷ 0.60 = 117
Moderate impairment range
PaO₂ 55 on FiO₂ 1.00
55 ÷ 1.00 = 55
Severe impairment range
Why the P/F Ratio Matters Clinically
The P/F ratio helps RTs and healthcare teams trend oxygenation over time. It is especially useful when FiO₂ changes, because PaO₂ alone can be misleading.
| Clinical Use | Why It Helps |
|---|---|
| Trending oxygenation | Shows whether oxygenation is improving or worsening after FiO₂ changes. |
| ARDS severity framing | Helps categorize oxygenation impairment when ARDS criteria are present. |
| Clinical communication | Provides a quick shared number for oxygenation severity. |
| Ventilator management context | May support discussions about PEEP, FiO₂, prone positioning, or escalation strategies. |
Common Student Mistakes
Do not calculate 80 ÷ 40. Use 80 ÷ 0.40.
A PaO₂ of 80 is very different on room air versus 100% oxygen.
P/F ratio helps classify severity but does not diagnose ARDS by itself.
A single value matters, but direction over time is often more clinically useful.
Practice Oxygenation and ABG Interpretation
Use the PulmoLearn ABG practice cases to strengthen your interpretation workflow, then continue into oxygenation calculations, hypoxemia mechanisms, and respiratory care decision-making.