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This is a detailed account of a method for the quantitative estimation of vitamin D, based on the roentgenographic examination of the healing of rickets in rats, for which an arbitrary scale of twelve different stages of healing is used. The curative effects of graded doses of the substance under examination are compared with the effects produced in a comparable series of animals by doses of a standard preparation, both being measured in terms of the roentgenographic scale described in this paper. Some of the factors taken account of in using this technic are breeding stock, age, weight, diet and caging of the young rats used for the test, and x-ray plant operated under specified conditions. A representative healing scale with detailed description is presented. Twelve roentgenograms comprise the series used in the scale, each picture being distinct enough to be distinguished from the others and to represent a definite
Quantitative Estimation of Vitamin D by Radiography.. JAMA. 1931;97(25):1916. doi:10.1001/jama.1931.02730250074048