A–a gradient

Imagine measuring the air pressure just outside a leaky tyre and again just inside it; a big difference tells you the tyre is failing to hold what it should. The A–a gradient applies the same logic to oxygen: it compares the oxygen pressure in the air sacs (the alveolar 'A') with the oxygen pressure actually achieved in the arterial blood (the arterial 'a'). A wide gap means oxygen is being lost somewhere between air and blood.

The alveolar value is calculated from a simple equation using the inhaled oxygen and the measured carbon dioxide, while the arterial value comes straight from a blood gas. In a healthy young adult breathing room air the difference is small — only a few mmHg, widening modestly with age. The number is useful because it separates two families of low blood oxygen: a normal gradient points to a problem outside the gas-exchange surface (too little breathing overall, or thin air at altitude), while a widened gradient points to a problem at the surface itself — mismatch, shunt or impaired diffusion.

So the A–a gradient is best thought of as an efficiency index of the lung as a gas exchanger, more informative than the oxygen level alone. A caveat: it must be interpreted in light of the inhaled oxygen, since supplemental oxygen and age both change the expected value, and the simple bedside formula assumes the person is breathing room air.