The Use and Interpretation of Assays for Vitamin D and its Metabolites

Abstract

Vitamin D is essential for the maintenance of calcium and bone metabolism in humans. The recommended daily allowance (RDA) for vitamin D in the United States of 200 KI (5.0 µg) is reasonable for adults who receive some exposure to sunlight; however, in the absence of any exposure to sunlight, this recommendation may be 2 to 3 times lower than that actually required to satisfy the body's needs. Vitamin D was first measured by bioassays. However, bioassays became obsolete in light of the revelation that vitamin D must be activated first in the liver to 25-hydroxyvitamin D (25-OH-D) and then in the kidney to 1,25-dihydroxyvitamin D [1,25(OH)₂D] before becoming biologically functional. Current assays measure circulating concentrations of vitamin D, 25-OH-D or 1,25(OH)₂D. The serum vitamin D concentration is of value for determining the role of sunlight in producing vitamin D in skin and as a provocative test to determine the absorption of vitamin D in patients with malabsorption syndromes. The serum concentration of 25-OH-D is most valuable for determining the overall vitamin D status of an individual, since it is an average of dietary and sunlight-induced vitamin D. The measurement of the serum 1,25(OH)₂D concentrations has been most useful in evaluating disorders in calcium and bone metabolism related to acquired and inborn errors in the conversion of 25-OH-D to 1,25(OH)₂D.