Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1285 page)

   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%.

   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.

VITAMIN B
₁
(THIAMINE)

   Definition

   Thiamine, first named “the antiberiberi factor” in 1926, has a historical value due to the very early description of beriberi in the Chinese medical texts, as far back as 2697
BC
. Thiamine is found in larger quantities in food products such as yeast, legumes, pork, rice, and cereals. Milk products, fruits, and vegetables are poor sources of thiamine. The thiamine molecule is denatured at high pH and high temperatures. Hence, cooking, baking, and canning of some foods as well as pasteurization can destroy thiamine. Thiamine is an essential vitamin required for carbohydrate metabolism, brain function, and peripheral nerve myelination. Thiamine deficiency has been associated with three disorders: Beriberi (infantile and adult), Wernicke-Korsakoff syndrome, and Leigh syndrome.

   
Normal range:
70–180 nmol/L.