🫁 A–a Oxygen Gradient Calculator
Calculate the alveolar–arterial oxygen gradient to characterise hypoxaemia.
Clinical takeaway
Widened gradient: V/Q mismatch, shunt or diffusion defect.
Loading calculator…
Paste the link in Slack, Teams, X, or LinkedIn — the preview image comes from this page’s Open Graph card.
When to use
Distinguish causes of hypoxaemia (shunt/V-Q mismatch vs hypoventilation).
How it works
PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 (sea level); A–a = PAO₂ − PaO₂. Expected ≈ age/4 + 4.
Key points
- Widened gradient: V/Q mismatch, shunt or diffusion defect.
- Normal gradient with hypoxaemia: hypoventilation or low inspired O₂.
- Expected value rises with age and FiO₂.
References
Decision support for licensed clinicians only; not a substitute for clinical judgement, diagnosis or local protocols.
Worked calculation
The values below come from this tool's own example placeholders and are computed server-side with the formula shown on this page, so the arithmetic can be checked quickly. It demonstrates how to substitute values only — it is not clinical advice and not a real case.
| PaO₂ (arterial) | 90 mmHg |
|---|---|
| PaCO₂ | 40 mmHg |
| FiO₂ | 0.21 fraction |
| Age (for expected) | 50 years |
→A–a gradient10mmHg
- Alveolar PAO₂：100 mmHg
- Formula：PAO₂ = FiO₂·(760−47) − PaCO₂/0.8 (sea level)
- Expected (age)：≈ 17 mmHg (age/4 + 4)
Frequently asked questions
- What is A–a Oxygen Gradient Calculator?
- Calculate the alveolar–arterial oxygen gradient to characterise hypoxaemia.
- How is A–a Oxygen Gradient Calculator calculated? What is the core formula?
- PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 (sea level); A–a = PAO₂ − PaO₂. Expected ≈ age/4 + 4.
- When is A–a Oxygen Gradient Calculator used?
- Distinguish causes of hypoxaemia (shunt/V-Q mismatch vs hypoventilation).
- What are the key clinical points for A–a Oxygen Gradient Calculator?
- Widened gradient: V/Q mismatch, shunt or diffusion defect. Normal gradient with hypoxaemia: hypoventilation or low inspired O₂. Expected value rises with age and FiO₂.
- What are the limits and cautions when using A–a Oxygen Gradient Calculator?
- For licensed clinicians and clinical researchers. Interpret results with history, investigations and local protocols; not a diagnosis or prescription, and not a substitute for multidisciplinary decision-making or local guidelines.
- How is A–a Oxygen Gradient Calculator calculated in practice? Can you show a worked example?
- Inputs: PaO₂ (arterial) 90 mmHg, PaCO₂ 40 mmHg, FiO₂ 0.21 fraction, Age (for expected) 50 years → Result: A–a gradient 10 mmHg（Alveolar PAO₂: 100 mmHg, Formula: PAO₂ = FiO₂·(760−47) − PaCO₂/0.8 (sea level), Expected (age): ≈ 17 mmHg (age/4 + 4)）