3-epi-25 hydroxyvitamin D concentrations are not correlated with age in a cohort of infants and adults

► A new LC-MS/MS assay resolves C-3-epi-25-hydroxyvitamin D from native analyte. ► Concentrations of C-3-epi-25-hydroxyvitamin D in infants and adults were determined. ► The concentration of epimer declines with age, but is still present in adults. ► The new assay correlates well with RIA, unlike many other LC-MS/MS methods.