What is the A-a gradient and why does it matter?

The A-a gradient is the difference between the oxygen pressure in the alveoli and in arterial blood. A normal gradient with low oxygen points to hypoventilation or low inspired oxygen, while a widened gradient signals a problem at the gas-exchange membrane such as V/Q mismatch, shunt, or a diffusion defect.

What is a normal A-a gradient?

On room air the expected gradient rises with age. A common bedside estimate is age divided by 4, plus 4, in mmHg. So a healthy 20-year-old sits near 9 mmHg while a 70-year-old may normally reach about 21 mmHg.

Which formula does this use?

It uses the alveolar gas equation: PAO2 = FiO2 times (atmospheric pressure minus 47) minus PaCO2 divided by 0.8. The 47 is water vapour pressure at body temperature and 0.8 is the respiratory quotient. The A-a gradient is then PAO2 minus the measured PaO2.

Does the age-expected range apply on supplemental oxygen?

Strictly, no. The age formula was derived on room air. On supplemental oxygen the gradient naturally widens, so the tool falls back to a rough 15 mmHg upper limit and you should interpret values in clinical context rather than against the age rule.

Is this a diagnostic tool?

No. It is an educational calculator that reproduces a standard equation. Clinical interpretation of any blood gas requires the full picture: the patient, the ventilator settings, and the trend over time. Always confirm management with a clinician.

Alveolar-Arterial (A-a) Gradient Calculator

The alveolar-arterial oxygen gradient measures how efficiently the lungs move oxygen from inhaled air into the bloodstream. It is one of the most useful single numbers in arterial blood gas interpretation because it separates causes of low blood oxygen that lie inside the lung from those that do not.

How it works

The gradient is the difference between the calculated oxygen pressure in the alveoli and the measured oxygen pressure in arterial blood:

PAO2     = FiO2 x (Patm - PH2O) - (PaCO2 / R)
A-a gap  = PAO2 - PaO2

Here PH2O is the water vapour pressure at body temperature, fixed at 47 mmHg, and R is the respiratory quotient, conventionally 0.8. The atmospheric pressure defaults to 760 mmHg at sea level and should be lowered at altitude.

The expected normal gradient widens with age. A widely used estimate is:

expected = (age / 4) + 4

Interpretation and notes

A low PaO2 with a normal gradient points to hypoventilation, which raises PaCO2 and lowers alveolar oxygen, or to a low inspired oxygen fraction such as at altitude. A low PaO2 with a widened gradient localises the problem to the gas-exchange surface: ventilation-perfusion mismatch (the most common cause), right-to-left shunt, or impaired diffusion. The age-expected range applies on room air only; on supplemental oxygen the gradient widens and should be read in clinical context.