Interpretation
| Result | Meaning |
|---|---|
| ≤ age/4 + 4 | Normal for age (room air) |
| > age/4 + 4 | Elevated: V/Q mismatch, shunt, diffusion defect |
Formula
PAO₂ = FiO₂ × (Patm − 47) − PaCO₂ ÷ 0.8; A–a gradient = PAO₂ − PaO₂; expected upper limit ≈ age/4 + 4 mmHg (room air). Patm 760 mmHg at sea level; 47 = water vapour pressure.
Clinical pearls
- The normal gradient widens on supplemental oxygen (up to ~100 mmHg on 100% O₂), so age norms apply only to room air.
- Hypoxaemia with a normal gradient = hypoventilation (high PaCO₂) or low inspired O₂ (altitude).
- Respiratory quotient 0.8 is assumed.
Examples
a-a gradient room air pao2 60 paco2 40 age 40aa gradient fio2 21 po2 90 pco2 40 40 yoA-a gradient on 40% oxygen, PaO2 9 kPa, PaCO2 5 kPa, 70 year old
FAQ
What is a normal A–a gradient?
On room air, under about 10–15 mmHg in young adults, rising with age; a common estimate of the upper limit is age/4 + 4 mmHg.
Why calculate the A–a gradient?
It separates hypoxaemia from hypoventilation (normal gradient) from lung causes such as V/Q mismatch, shunt and diffusion impairment (raised gradient).