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 � �g administered IV over 30 minutes or as 450 � �g 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%.
Use
Identify subjects with marginal liver vitamin A stores
As a tool for estimation of total body stores of vitamin A
Interpretation
Increased In
RDR values of >20% indicate depleted liver vitamin A stores.
Limitations
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.