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Test-Retest Reproducibility of the Relative Dose Response for Vitamin A Status in Guatemalan Adults: Issues of Diagnostic Sensitivity¹

Center for Studies of Sensory Impairment, Aging and Metabolism, the research branch of the National Committee for the Deaf and Blind of Guatemala, Hospital de Ojos and Oidos Dr. Rodolfo Robles V., Z11, Guatemala City, Guatemala ^* U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, MA 02111.

The relative dose response (RDR) test has been used as a functional measure of whole-body stores of vitamin A in humans. We have examined the reproducibility of the RDR procedure in a population of Guatemalan adult subjects who would be expected to show a moderate prevalence of hypovitaminosis A. Fifty-one subjects were administered a standard RDR test, and the plasma samples were analyzed for retinol, tocopherol, retinol binding protein (RBP) and prealbumin (PAL). Thirty-four of the subjects underwent repeat RDR tests 7 d later. Plasma levels in fasted subjects were as follows: retinol, 1.35 ± 0.30 µmol/L; RBP, 37.8 ± 7.7 mg/L; PAL, 187.0 ± 39.0 mg/L; and tocopherol, 16.6 ± 6.2 µmol/L. RDRs ranged from -35.2% to +63.1%, with a mean of 2.6 ± 10.4%. Overall, we observed poor within-subject reproducibility of the RDR procedure whether expressed numerically or by diagnostic classification. Moreover, in contrast to previous studies in children, we observed fewer positive RDR tests than would be expected for the population studied. Nevertheless, the mean RDR was inversely proportional to fasting retinol levels, thus confirming the validity of the biological basis of the RDR procedure in humans. Because of high intra-individual variability with this test, investigators should be cautious when using the RDR procedure in serial studies to monitor the efficacy of therapeutic interventions or subject compliance to dietary regimens.

KEY WORDS: • vitamin A • retinol • relative dose response • retinol binding protein • humans

¹ Supported in part by U.S. Department of Agriculture Agricultural Research Service contract 53-3K06-5-10.

² Requests for reprints should be addressed to R. M. Russell, USDA Human Nutrition Research Center on Aging at Tufts University, 711 Washington Street, Boston, MA 02111.

Manuscript received 27 September 1989. Revision accepted 21 February 1990.