A–a Oxygen Gradient
Is hypoxaemia due to a gas-exchange problem or hypoventilation?
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About this tool
Alveolar gas equation and A–a gradient with comparison to the age-expected value.
Intended use
Patients with an arterial blood gas, ideally on room air.
Outcome
Alveolar–arterial oxygen difference
Time horizon
Point in time
Limitations
- Normal gradient rises with FiO₂ and age.
- Assumes RQ 0.8.
Clinical problems
Evidence
Evidence summaries for this tool are awaiting independent clinical review. Statistics are shown only where they could be attributed to a specific publication.
References
- Review / updateMellemgaard K. The alveolar-arterial oxygen difference: its size and components in normal man. Acta Physiol Scand 1966;67:10-20. Find on PubMed (opens in a new tab)
Version history
| Version | Formula | Effective | Reviewer | Changes |
|---|---|---|---|---|
| v1.0.0 | Alveolar gas equation (RQ 0.8, PH2O 47 mmHg) | 2026-10-03 | — | Initial implementation with reference test vectors. |
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Clinical decision support only. Interpret results in the context of the individual patient and current guidance; HumanPulse does not replace professional medical judgment.