P/F Ratio Calculator
The P/F Ratio Calculator estimates the PaO₂/FiO₂ ratio using arterial oxygen tension and fraction of inspired oxygen. This tool helps medical professionals quickly assess lung function and diagnose ARDS severity. Whether you are managing a patient on mechanical ventilation or evaluating hypoxemia in the ICU, this calculator provides reliable, immediate results.
How P/F Ratio Is Calculated
The P/F Ratio measures how well oxygen moves from the lungs into the bloodstream. It compares the oxygen level in the blood (PaO₂) to the amount of oxygen a patient is breathing (FiO₂).
P/F Ratio = PaO₂ ÷ (FiO₂ / 100)
Where:
- PaO₂ = Arterial oxygen tension (in mmHg)
- FiO₂ = Fraction of inspired oxygen (entered as a percentage, e.g., 50)
First, obtain the PaO₂ value from an arterial blood gas (ABG) test. Next, convert the FiO₂ percentage to a decimal by dividing by 100. Finally, divide the PaO₂ by the decimal FiO₂ to get the ratio. This calculation shows how efficiently the lungs transfer oxygen. A lower number means the lungs are struggling, even with high oxygen support. This method is a standard, reliable indicator used in critical care.
What Your P/F Ratio Means
Your P/F Ratio score helps classify the severity of acute respiratory distress syndrome (ARDS). It guides treatment decisions by showing how much support the lungs need.
No ARDS: A ratio greater than 300 indicates normal lung function or mild hypoxemia. Patients in this range typically do not meet the criteria for ARDS.
Mild ARDS: A ratio between 201 and 300 suggests mild lung injury. Monitor the patient closely, as they may require supplemental oxygen but often do not need intubation.
Moderate ARDS: A ratio between 101 and 200 indicates moderate severity. These patients usually need positive pressure ventilation and higher levels of PEEP (positive end-expiratory pressure).
Severe ARDS: A ratio of 100 or less signifies severe lung failure. This requires immediate, aggressive intervention, often including prone positioning and advanced mechanical ventilation strategies.
Important: Always ensure the ABG sample is drawn after the patient has been on a stable oxygen setting for at least 15 to 30 minutes to avoid inaccurate readings.
*This calculator is a screening tool for educational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider for diagnosis and treatment decisions.*
Recalculate the ratio regularly, especially after changing ventilator settings, to track the patient's progress and response to therapy.