Analytical measurement of serum 25-OH-vitamin D3, 25-OH-vitamin D2 and their C3-epimers by LC-MS/MS in infant and pediatric specimens

► A new LC-MS/MS developed for the quantification of serum 25-OH vitamin D metabolites. ► Serum 25-OH-vitamin D3 increased in the first year, while C3-epimer remained stable. ► C3-epi-25-OH-vitamin D3 concentrations decreased in children older than one year. ► Over-the-counter vitamin D3 supplements did not contain the C3-epimer form.