Alveolar–arterial gradient

The Alveolar–arterial gradient (A-aO₂, or A–a gradient), is a measure of the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen. It is used in diagnosing the source of hypoxemia.
It helps to assess the integrity of alveolar capillary unit. For example, in high altitude, the arterial oxygen is low but only because the alveolar oxygen (PAO₂) is also low. However, in states of ventilation perfusion mismatch, such as pulmonary embolism or right-to-left shunt, oxygen is not effectively transferred from the alveoli to the blood which results in elevated A-a gradient