Interlaboratory comparison of 25-hydroxyvitamin D determination.

This interlaboratory study on determination of 25-hydroxyvitamin D (25-OH-D) in serum involved 15 laboratories in eight European countries. All could distinguish between normal (50 +/- 31 nmol/L, mean +/- SD) and grossly increased concentrations, but for eight laboratories the results for serum samples with low and normal 25-OH-D content overlapped. In general, values were well reproducible, but interlaboratory variation in 25-OH-D measurement was large, 24,25(OH)2D3 interfering in most of the assays. We present evidence in favor of chromatography before assay, as opposed to nonchromatographic methods. Liquid chromatography with ultraviolet detection for quantifying 25-OH-D2 and 25-OH-D3 appears to be an appropriate reference method, whereas competitive protein binding assay is the method of choice for routine determinations. Control sera with subnormal, normal, and above-normal concentrations of 25-OH-D3 are needed for use in standardization of 25-OH-D assays.