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Subject: Vitamin A Relative Dose–Response (RDR) Test
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Retinol (vitamin A) measurements in blood have several disadvantages. It is decreased only in severe vitamin A deficiency, when liver stores are nearly exhausted. In addition, infection can decrease serum vitamin A levels in blood, leading to misclassification of individuals. Because the majority of vitamin A is stored in liver, RDR test/calculation was developed to reliably measure vitamin A storage. Retinyl palmitate is given either as a water miscible solution of 1,000 μg administered IV over 30 minutes or as 450 μg diluted in corn oil and administered orally. Fasting and 5 hours after-dose plasma specimens should be drawn. RDR is calculated as 5-hourvitamin A fasting (0 hour)/vitamin A 5 hour × 100.
Normal range: <10%.
Identify subjects with marginal liver vitamin A stores
As a tool for estimation of total body stores of vitamin A
RDR values of >20% indicate depleted liver vitamin A stores.
The vitamin A RDR oral dose test has the similar limitations of other absorption tests. It decreases in malabsorption, cirrhosis, cholestasis, hepatocellular disease, protein–calorie malnutrition, and zinc deficiency.