A-a Gradient Calculator
Calculate the alveolar-arterial (A-a) oxygen gradient to evaluate oxygenation and differentiate causes of hypoxemia.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
Inputs
Normal PaO₂: 80–100 mmHg (room air); mild hypoxemia 60–79; moderate 40–59; severe <40 mmHg
Normal PaCO₂: 35–45 mmHg; <35 resp. alkalosis; >45 resp. acidosis; used in alveolar gas equation
Results
A-a Gradient
4.7mmHg
Elevated
0(0=No, 1=Yes)
How to Use This Calculator
- Enter FiO₂, PaO₂, PaCO₂, atmospheric pressure, and patient age.
- Review the A-a gradient — normal is < age/4 + 4 mmHg on room air.
- An elevated A-a gradient suggests a parenchymal, vascular, or diffusion lung abnormality.
How the result changes with Atmospheric Pressure
| Atmospheric Pressure | A-a Gradient | Elevated |
|---|---|---|
| 440 | -62.5mmHg | 0(0=No, 1=Yes) |
| 540 | -41.5mmHg | 0(0=No, 1=Yes) |
| 660 | -16.3mmHg | 0(0=No, 1=Yes) |
| 760 | 4.7mmHg | 0(0=No, 1=Yes) |
What each input means
- FiO₂
- Fraction of inspired oxygen as a percentage. Room air = 21%.
- PaO₂
- Arterial partial pressure of oxygen from arterial blood gas (ABG). Normal range: 80–100 mmHg on room air (age-adjusted: ≈104 − 0.27×age mmHg). Mild hypoxemia: 60–79 mmHg; moderate: 40–59; severe: <40 mmHg.
- PaCO₂
- Arterial partial pressure of carbon dioxide from ABG. Normal range: 35–45 mmHg. Respiratory alkalosis: <35 mmHg (hyperventilation); respiratory acidosis: >45 mmHg (hypoventilation). Used in alveolar gas equation: PAO₂ = FiO₂×(Patm−47) − PaCO₂/0.8.
- Atmospheric Pressure
- Barometric pressure. Sea level = 760 mmHg. Adjust for altitude.
- Patient Age
- Patient age for calculating expected A-a gradient.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersFiO₂ = 21, PaO₂ = 95, PaCO₂ = 40, Atmospheric Pressure = 760 = 5 input(s) provided
- Calculate A-a GradientA-a Gradient4.7 = 4.7
- Calculate ElevatedElevated0 = 0
- Calculate Expected GradientExpected Gradient14 = 14
- Calculate Alveolar PO₂Alveolar PO₂99.7 = 99.7
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