A–a oxygen gradient
Alveolar–arterial O₂ difference from a blood gas, in kPa (mmHg input accepted)
Sources last checked: 2026-10-13
A–a oxygen gradient
Alveolar–arterial O₂ difference from a blood gas, in kPa (mmHg input accepted)
Assumes sea-level barometric pressure and RQ 0.8; at altitude the gradient is overestimated. Age norms apply to room air only. Choose the unit first — values implausible for that unit are rejected.
Interpretation
| < 1.1 kPa (< 8.5 mmHg) | Within the age-expected value at any adult age when breathing room air. |
|---|---|
| 1.1–3.8 kPa — compare with age | Age-expected on room air (age/4 + 4 mmHg): 20 y ≈ 1.2 kPa, 40 y ≈ 1.9 kPa, 60 y ≈ 2.5 kPa, 80 y ≈ 3.2 kPa. Above that for the patient's age = raised gradient. |
| ≥ 3.9 kPa (≥ 29 mmHg) | Above the age-expected value at any adult age on room air: V/Q mismatch, shunt or diffusion limitation. On supplemental oxygen the gradient rises with FiO₂ and age norms no longer apply. |
What is the A–a oxygen gradient?
The alveolar–arterial (A–a) oxygen gradient is the difference between the calculated alveolar oxygen tension (from the simplified alveolar gas equation) and the measured arterial PaO₂. It separates hypoxaemia due to hypoventilation or low inspired oxygen (normal gradient) from hypoxaemia due to V/Q mismatch, shunt or diffusion limitation (raised gradient). The normal gradient widens with age; Mellemgaard's classic data in healthy adults give a mean of about 2.5 + 0.21 × age mmHg.
When to use it
- Working up unexplained hypoxaemia from an arterial blood gas
- Distinguishing pure hypoventilation (e.g. opioid toxicity, neuromuscular weakness) from a lung parenchymal or vascular cause
- Teaching blood-gas interpretation with kPa values as reported in most European laboratories
When not to use it
- Interpreting against age norms when the patient is on supplemental oxygen — the gradient rises with FiO₂
- At altitude without correcting barometric pressure
- Venous or capillary samples
How it is calculated
PAO₂ = FiO₂ × (Pb − PH₂O) − PaCO₂ / 0.8 Pb = 101.3 kPa (760 mmHg), PH₂O = 6.3 kPa (47 mmHg), RQ = 0.8 A–a gradient = PAO₂ − PaO₂ (result in kPa; mmHg input ÷ 7.5) Age-expected on room air ≈ (age/4 + 4) mmHg; Mellemgaard mean 2.5 + 0.21 × age mmHg
Clinical pearls and limitations
- Assumes sea-level barometric pressure and RQ 0.8; at altitude the gradient is overestimated. Age norms apply to room air only. Choose the unit first — values implausible for that unit are rejected.
- A normal gradient with high PaCO₂ points to hypoventilation; a raised gradient points to a lung problem.
- In kPa the age rule of thumb (age/4 + 4 mmHg) becomes roughly (age/4 + 4) ÷ 7.5 kPa — about 2.5 kPa at 60 years.
- FiO₂ delivered by masks and nasal cannulae is uncertain, so gradients on low-flow oxygen are only approximate.
- The RQ of 0.8 is an assumption; it changes with diet and metabolism and slightly alters the result.
Frequently asked questions
What is a normal A–a gradient in kPa?
On room air roughly up to 1–2 kPa in young adults, rising with age to about 3 kPa at 80 years by the common (age/4 + 4) mmHg estimate.
Which formula does this calculator use?
PAO₂ = FiO₂ × (barometric pressure − water vapour pressure) − PaCO₂/0.8, with sea-level pressure 101.3 kPa (760 mmHg) and water vapour 6.3 kPa (47 mmHg); the gradient is PAO₂ − PaO₂.
Why is the result shown in kPa?
Most European laboratories report blood gases in kPa. You can still enter mmHg — choose the unit first — and the result is converted with 1 kPa = 7.50 mmHg.
Can the A–a gradient be negative?
Not physiologically. A negative result usually means the FiO₂, PaCO₂ or PaO₂ entry is wrong, or the sample is mislabelled.
Evidence and guidelines
- Primary source: Mellemgaard K. The alveolar-arterial oxygen difference: its size and components in normal man. Acta Physiol Scand 1966;67(1):10-20 — as summarised with the simplified alveolar gas equation in Murray & Nadel's Textbook of Respiratory Medicine, 6th ed., chapter 'Pulmonary Function Testing' (open access via PMC7158317).
- Age-expected gradient (age/4 + 4) mmHg as used in: Alveolar arterial gradient and respiratory index in predicting the outcome of COVID-19 patients. Arch Acad Emerg Med 2022;10(1):e28.
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