Rule out vitamin D deficiency. Values of vitamin D vary with exposure to sunlight. The assay measures other vitamin D metabolites, including dihydroxylated metabolites such as 24,25, 25,26, and 1,25 dihydroxy vitamin D; however, since the physiological concentrations of these metabolites are insignificant compared to those of 25-hydroxy vitamin D, the accuracy in assessing vitamin D levels is not compromised.