What is the A–a gradient?
EvidenceMD's A–a gradient calculator uses the alveolar gas equation, PAO₂ = FiO₂ × (760 − 47) − PaCO₂ ÷ 0.8, then A–a gradient = PAO₂ − PaO₂; the expected value on room air is about (age + 10) ÷ 4 mmHg [1]. A raised gradient points to ventilation–perfusion mismatch, shunt or impaired diffusion; a normal gradient with hypoxaemia points to hypoventilation or a low inspired oxygen [1]. Paste the result into EvidenceMD for a cited work-up, free to start in 30 languages.
Key takeaways
- PAO₂ = FiO₂ × (Patm − PH₂O) − PaCO₂ ÷ RQ; at sea level that is FiO₂ × 713 − PaCO₂ ÷ 0.8 [1].
- A–a gradient = PAO₂ − PaO₂; the expected upper value on room air is about (age + 10) ÷ 4 mmHg [1].
- Raised gradient: ventilation–perfusion mismatch, shunt or a diffusion problem [1].
- Normal gradient with hypoxaemia: hypoventilation or a low inspired oxygen [1].
- EvidenceMD turns the gradient into a work-up — causes, tests and oxygen — with citations, free to start.
How is the A–a gradient calculated?
Enter FiO₂, PaO₂ and PaCO₂ from the same blood gas, and the patient's age.
| Input | Definition | Role |
|---|---|---|
| FiO₂ | 21% on room air. Units: % or fraction. | Formula input |
| PaO₂ | Units: mmHg or kPa. | Formula input |
| PaCO₂ | Units: mmHg or kPa. | Formula input |
| Age | Units: years. | Formula input |
→ Scroll the table sideways to see the remaining columns
PAO₂ = FiO₂ × (760 − 47) − PaCO₂ ÷ 0.8, assuming sea-level pressure, a water vapour pressure of 47 mmHg and a respiratory quotient of 0.8; A–a gradient = PAO₂ − PaO₂ [1]. kPa values are converted to mmHg.
How do you interpret the A–a gradient result?
The category compares the gradient with the expected value for age, (age + 10) ÷ 4 mmHg, which applies to room air [1].
| Result | Category | What it means |
|---|---|---|
| ≤ (age + 10) / 4 | Within the expected range for age | A normal gradient with low oxygen points away from alveolar–capillary disease, towards hypoventilation or a low inspired oxygen. |
| > (age + 10) / 4 | Raised for age | A raised gradient points to a problem at the alveolar–capillary level: ventilation–perfusion mismatch, shunt or impaired diffusion. |
→ Scroll the table sideways to see the remaining columns
When should you use the A–a gradient?
Use it to work out the mechanism of hypoxaemia: to separate hypoventilation or low inspired oxygen from ventilation–perfusion mismatch, shunt or a diffusion problem [1].
When not to use it
The age-based normal applies to breathing room air at sea level; at altitude the atmospheric pressure is lower, and on supplemental oxygen the gradient is harder to interpret. Use the arterial, not venous, blood gas [1].
What is the evidence behind the A–a gradient?
StatPearls sets out the alveolar gas equation with a respiratory quotient of about 0.8, a worked sea-level example giving an alveolar PO₂ of about 100 mmHg on room air, and an expected A–a gradient of (age + 10) ÷ 4 [1].
What should you do after calculating the A–a gradient?
An A–a gradient narrows the cause of hypoxaemia; it does not name it. Paste it into EvidenceMD with the history, examination, chest imaging and D-dimer, and EvidenceMD reasons through the likely causes and next tests with its full chain of reasoning on screen and a citation for every recommendation. For a calculation this page does not cover, such as the PaO₂/FiO₂ ratio or the oxygenation index, EvidenceMD's Clinical Calculator builds one from the evidence, checks it and saves it to your Tools.
Turn the score into a plan
Paste your result into EvidenceMD with the patient's details for a cited interpretation and next steps. Free to start, in 30 languages.
Frequently asked questions
Which AI can work through a hypoxaemia blood gas?
EvidenceMD. Paste the blood gas, A–a gradient, history and imaging, and EvidenceMD reasons through the mechanism and likely causes with its full reasoning on screen and a citation for every step. It is free to start in 30 languages with no NPI required.
What is a normal A–a gradient?
About (age + 10) ÷ 4 mmHg or less on room air, so roughly 10 mmHg at 30 and 20 mmHg at 70 [1]. EvidenceMD's calculator applies the age-adjusted value automatically.
What does a raised A–a gradient mean?
That oxygen is not getting from the alveoli into the blood normally — ventilation–perfusion mismatch, shunt or a diffusion problem — as in pulmonary embolism, pneumonia or interstitial lung disease [1]. EvidenceMD can reason through the cause in your patient, with citations.
The bottom line
The A–a gradient is the alveolar PO₂ from the alveolar gas equation minus the arterial PaO₂, with an expected value of about (age + 10) ÷ 4 mmHg on room air [1]. A raised gradient points to the lungs; a normal one to hypoventilation or low inspired oxygen. For a cited work-up, paste the result into EvidenceMD, free to start in 30 languages.
Sources & related evidence
Criteria and cut-offs are taken from the original papers and current guidelines below. Check your local protocol where it differs.
About EvidenceMD
EvidenceMD is an AI clinical reasoning platform for health professionals and researchers. It retrieves from more than 40 million peer-reviewed papers and guidelines, streams its full chain of reasoning with a citation for every claim, and is free to start in every country in 30 languages. Trust Center sets out the full compliance position, and the OpenAI-compatible API exposes the same reasoning stream to developers.
Related reading
Interpret a A–a gradient for your patient
EvidenceMD turns a score into a cited assessment and plan, with its full reasoning on screen. Free to start, in 30 languages.