Alveolar/Arterial Gradient and Arterial/Alveolar Oxygen Ratio


Alveolar-arterial difference, A/a gradient, a/A ratio.

To assist in assessing oxygen delivery and diagnosing causes of hypoxemia, such as pulmonary edema, acute respiratory distress syndrome, and pulmonary fibrosis.

Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction. Indicate the type of oxygen, mode of oxygen delivery, and delivery rate as part of the test requisition process. Wait 30 min after a change in type or mode of oxygen delivery or rate for specimen collection.

Normal Findings
Method: Selective electrodes that measure Po2 and Pco2.

Alveolar/arterial gradientLess than 10 mm Hg at rest (room air); 20–30 mm Hg at maximum exercise activity (room air)
Arterial/alveolar oxygen ratioGreater than 0.75 (75%)
Values normally increase with increasing age (see study titled “Blood Gases”).

Critical Findings and Potential Interventions

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