Aa Gradient = PAO2 - PaO2
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. PAO2: Alveolar PO2 (calculated from the alveolar gas equation). PaO2: Arterial PO2 (measured in arterial blood).
Adrian J Williams: Assessing and interpreting arterial blood gaeses and acid-base balances. BMJ 1998 Oct 31. Web. 18 July 2016.