Alveolar-arterial Oxygen Gradient is expressed in mmHg (torr).
PA-aO2 and A-aDO2 are used indistinctly.
PH2O, PB and R field values have been prepopulated for convenience, they can be overwritten as needed.

* FIO2 can be entered as a % or fraction. L/min conversion can be done as follows:
FIO2 L/min = (FIO2 % - 21)/3 (only if FIO2 is > 21%). So for values below 21% the normal values for L/min don't apply.

Increased gradient: diffusion defect, Right-To-Left shunt (intrapulmonary or intracardiac), V/Q mismatch.
Diffusion defect increases gradient on 15% O2 but not on 100% O2.
To rule out R to L shunt (right to left) , administer O2 for 10 or 15 minutes, then recalculate A-aDO2. In general, diffusion defects and mild to moderate V/Q defects correct (at least partially). Shunts do not correct.