The Relative Dose Response RDR Test

3 The Relative Dose Response (RDR) Test The RDR test is a test of the ability of a dose of vittunin A to reuse the plasma concentration of retinol severed hours later, eifter chjdomicrons have been cleeired from the circulation. What is being tested is the ability of the Uver to releeise retinol into the circulation. In subjects who etre retinol deficient, a test dose will produce a leirge increase in pleisma retinol, because of the accumulation of apo-RBP in the liver in deficiency (Section 2.2.3). In those whose problem is due to lack of RBP, then little of the dose will be releeised into the circulation. An RDR greater them 20% indicates depletion of liver reserves of retinol to less them 70 /imol per kg (Underwood, 1990). 

The test requires two samples of blood, taken before and 5 hours after the test dose of retinol. A modified RDR test involves giving a test dose of dehydroretinol, then determining the ratio of dehydroretinokretinol in a single plasma sample taken 30 hours later. Again, because of the accumulation of RBP in the liver in deficiency and because in deficiency there is less dilution of dehydroretinol with liver pools of retinyl esters, the ratio is inversely proportional to the liver stores of retinol (Tanumihardjo et al., 1987). 

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During the development of vitamin A deficiency in experimental animals, the plasma concentration of RBP falls, while the liver content rises. The administration of retinol to deficient animals results in a considerable release of holo-RBP from the liver. This is a rapid effect on the release of preformed apo-RBP in response to the availability of retinol, rather than an increase in the synthesis of the protein. There is no evidence that retinol controls the synthesis of RB P (Soprano et al., 1982). This provides the basis of the relative dose response (RDR) test for liver stores of vitamin A (Section 2.4.1.3) administration of a test dose of retinol gives a considerably greater increase in plasma retinol, bound to RBP, in deficient subjects than in those with adequate liver reserves, because of the accumulation of apo-RBP in the liver. 

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