A-a O2 Gradient

A-a gradient = PAO2 − PaO2[2]
Where:
PAO2 = alveolar PO2 (calculated from the alveolar gas equation)
PaO2 = arterial PO2 (measured in arterial blood A-a gradient)
In general the A-a gradient can be calculated by:
Aa Gradient = [FiO2*(Patm-PH2O)-(PaCO2/0.8) ] - PaO2
On Room air ( 21 % ) and at sea level, a simplified version of the equation is:
Aa Gradient = (150 - 5/4(PCO2)) - PaO2