Methods for assessment of vitamin A status.

Assessment of the relative level of vitamin A nutriture in human populations when clinical signs and symptoms of deficiency or of toxicity are absent has presented methodological difficulties. Commonly used indicators include dietary intakes of the vitamin, serum levels, and dark adaptation, all of which have limitations in their precision, especially when applied to individuals and to young children. New and developing nonclinical indicators include the relative dose response (RDR) test or a modification (MRDR) of it, conjunctival impression cytology (CIC), and isotope dilution to estimate total body reserves. These methods, all of which are promising, require additional work to verify their sensitivity, specificity, and predictive power as indicators of relative states of vitamin A depletion for individuals and populations under the many varied conditions commonly associated with an inadequate vitamin status. Currently, the most reliable assessment of vitamin A nutriture is likely when a combination of methods is used.