The A-a O₂ Gradient is the difference between alveolar oxygen tension and arterial oxygen tension. It helps distinguish causes of hypoxemia by indicating whether the problem is due to inadequate ventilation or impaired gas transfer within the lungs. PAO₂ is estimated with the alveolar gas equation and PaO₂ is measured by arterial blood gas analysis. A normal gradient in the setting of low PaO₂ ​ points to hypoventilation or low FiO₂​, while a widened gradient supports intrinsic pulmonary causes such as V/Q mismatch, diffusion impairment, or true shunt. Typical conditions include asthma, COPD, interstitial lung disease, pneumonia, pulmonary edema, ARDS, and pulmonary embolism. Although a high gradient is nonspecific, it correlates with the severity of gas exchange impairment and can complement imaging and biomarkers in disorders like pulmonary embolism.