Studies on the application of the relative-dose-response test for assessing vitamin A status in older adults.

We investigated the time course and the reproducibility of the relative-dose-response (RDR) test for assessing vitamin A status in older adults. The maximum plasma retinol response to 480 retinol equivalents (RE) of retinyl palmitate in abnormal responses was at 6 or 7 h after dosing compared with the 5-h sampling interval recommended by others for younger adults and children. With respect to reproducibility, the diagnostic concordance of two RDR tests at 7-d intervals in 14 elders was 71%. In 29% of tests, one test was abnormal and the other normal. Linear regression of the two RDR values in these 14 subjects gave a correlation coefficient of -0.08. We conclude that the procedure for the RDR should be modified when applied to persons greater than 60 y of age, and that multiple repetitions of the test are needed to provide a stable indication of vitamin A stores in an elderly individual.