A. Calculation of alveolar-arterial oxygen difference

  • Specimens for arterial blood gas analysis are drawn while patient is breathing room air (Flo2 = 21%).

  • The following formula is used to determine alveolar-to-arterial oxygen difference:

    [A-a]DO2 = 150 - 1.2(Paco2) - Pao2

B. Grading severity of PCP by oxygenation
Severity[A-a]Do2(mm Hg)Pao2(mm Hg)
Mild< 35> 70
Moderate35 to 45> 70
Severe> 45> 50