During the past three years, a convenient method of estimating vitamin A in small quantities of blood has been developed and applied to the problems of early clinical diagnosis of deficiency of vitamin A and the metabolism of vitamin A in health and in disease.
The clinical diagnosis of vitamin A deficiency, before the development of the characteristic keratomalacia, has been dependent on the recognition of early signs, as described by Blackfan and Wolbach.1 They pointed out that the most dependable early sign is the appearance of cornified epithelial cells in sites where they do not normally occur, for example, nose, trachea, kidneys and vagina.2 Yet even this evidence of deficiency of vitamin A may be considered a relatively late sign, as it appears only after weeks or months of almost complete deprivation of vitamin A. Quantitative measurement of night blindness has been proposed as an indirect approach
MAY CD, BLACKFAN KD, McCREARY JF, ALLEN FH. CLINICAL STUDIES OF VITAMIN A IN INFANTS AND IN CHILDREN. Am J Dis Child. 1940;59(6):1167-1184. doi:10.1001/archpedi.1940.01990170003001