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A–a Gradient Calculator
The alveolar–arterial (A–a) gradient compares the oxygen level expected in the alveoli with the oxygen actually measured in arterial blood. The alveolar value comes from the alveolar gas equation; a wider-than-expected gradient points toward a gas-exchange problem in the lungs (such as V/Q mismatch, shunt, or diffusion impairment) rather than pure hypoventilation.
Enter the values or tick the options above — the result updates automatically.
How to interpret the result
The A–a gradient normally widens with age. A widely used bedside estimate of the expected value is age ÷ 4 + 4 mmHg on room air. Reading the result:
| Pattern | Typical meaning |
|---|---|
| Gradient near the age-expected value, low PaO2 | Hypoventilation (for example opioid effect or neuromuscular weakness) or low inspired oxygen — the lungs themselves transfer oxygen normally. |
| Gradient wider than expected | A gas-exchange defect: V/Q mismatch (the commonest), right-to-left shunt, or diffusion impairment. |
This calculator assumes sea-level barometric pressure. At altitude the alveolar oxygen is lower, and the expected gradient changes.
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Frequently asked questions
What is a normal A–a gradient?
On room air it is roughly 5–15 mmHg in a young adult and rises with age; a common estimate is age divided by 4 plus 4. It also rises as the inspired oxygen fraction increases, so values on supplemental oxygen are not directly comparable with room-air values.
Why does the alveolar gas equation subtract 47?
47 mmHg is the partial pressure of water vapour in the airways at body temperature. Inspired gas is fully humidified, so water vapour takes up part of the total pressure before oxygen and carbon dioxide are considered.
What does a wide A–a gradient with a normal PaCO2 suggest?
That the lungs are the problem — classically V/Q mismatch, shunt, or diffusion impairment — rather than under-breathing, because pure hypoventilation raises PaCO2 and lowers PaO2 while keeping the gradient near normal.