Three commercial kits and one liquid-chromatographic method evaluated for determining 25-hydroxyvitamin D3 in serum.

The accuracy of three commercial kit methods and one liquid-chromatographic (HPLC) method for determining 25-hydroxyvitamin D3 in serum was evaluated by using isotope dilution-mass spectrometry as a reference technique. The kit methods were based on radioimmunoassay and radioreceptor assay. Serum samples were analyzed from male and female volunteers, some of whom had been exposed to ultraviolet B light or given vitamin D3 orally. Results obtained with the three commercial kits were less accurate than those by HPLC. The agreement between the HPLC method and the comparison method was relatively good: r = 0.99; slope = 1.23; intercept = -2.9 micrograms/L. We conclude that methods based on HPLC should be preferred in routine clinical work and that the accuracy of the three commercial kits tested is not sufficient for their intended use.