Multicenter comparison study of current methods to measure 25-hydroxyvitamin D in serum

► Ten methods to measure serum 25-hydroxyvitamin D were compared with ID-LC-MS/MS. ► Significant bias exists between ID-LC-MS/MS and many, but not all, other 25(OH)D methods. ► The response among different assays for 25(OH)D2 and 3-epi-25(OH)D3 is variable. ► This variable response impedes the use of uniform cut-off values for vitamin D status. ► Our results indicate the need towards further standardizing 25(OH)D assays.