P_(A-a)O₂ = (760-47)*FiO₂-(PaCO₂/0.8)-PaO₂

Normal value = (Age / 4) + 4. Generally between 15-20 mmHg, increases with age, usually does not exceed 30 mmHg.

A-a gradient, P_(A-a)O₂

FiO₂ on room air is 21%. An elevated A-a gradient indicates V/Q mismatch (e.g., COPD, interstitial lung disease, pulmonary vascular disease), diffusion impairment (e.g., interstitial lung disease or other causes of pulmonary inflammation/fibrosis), or right-to-left shunt (e.g., arteriovenous malformation, hepatopulmonary syndrome, atelectasis).

Normal A-a gradient hypoxemia suggests hypoventilation (e.g., COPD, central nervous system disease, neuromuscular disease) or low inspired oxygen concentration (e.g., high altitude).