Calculate the alveolar-arterial oxygen gradient to assess lung function.
ABG & Clinical Data
years
%
mmHg
mmHg
Oxygenation Result
Calculated Gradient4.7mmHg
Expected Normal14.0mmHg
Normal Gradient
Oxygen Transfer Efficiency
About this calculator
Overview
The A-a Gradient (Alveolar-arterial Gradient) is a measure of the difference between the alveolar concentration of oxygen and the arterial concentration of oxygen. It is a useful clinical tool for differentiating causes of hypoxemia (low blood oxygen).
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Pro Tips
Age-adjusted normal gradient ≈ (Age / 4) + 4.
Ensure PaO2 and PaCO2 are from the same arterial blood gas (ABG) sample.
FiO₂ for room air is 21%.
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Fun Facts
"A normal A-a gradient increases with age."
"A widened gradient suggests a problem with oxygen transfer (e.g., V/Q mismatch or shunt)."
"A normal gradient in a hypoxemic patient suggests hypoventilation or low inspired oxygen (altitude)."
"At sea level, Patm is 760 mmHg and PH2O is 47 mmHg."
Formula: PAO2 = [FiO2 × (Patm - PH2O)] - (PaCO2 / R) where R is Respiratory Quotient (~0.8). Gradient = PAO2 - PaO2.
