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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.
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.
Method: Selective electrodes that measure Po2 and Pco2.
|Alveolar/arterial gradient||Less than 10 mm Hg at rest (room air); 20–30 mm Hg at maximum exercise activity (room air)|
|Arterial/alveolar oxygen ratio||Greater than 0.75 (75%)|
|Values normally increase with increasing age (see study titled "Blood Gases").|
Critical Findings and Potential Interventions