Development and validation of the simultaneous measurement of four vitamin D metabolites in serum by LC-MS/MS for clinical laboratory applications.

The quantification of serum 25-hydroxyvitamin D [25(OH)D] as an indicator of vitamin D status is currently primarily conducted by immunoassays, yet LC-MS/MS would allow more accurate determination. Furthermore, LC-MS/MS would allow simultaneous measurement of multiple analytes. The aim of this study was to develop and validate an LC-MS/MS method to simultaneously measure four vitamin D metabolites (25(OH)D3, 3-epi-25(OH)D3, 25(OH)D2, and 24,25(OH)2D3) in serum for clinical laboratory applications. Serum samples were first prepared in a 96-well supported liquid extraction plate and the eluate was derivatized using the Cookson-type reagent 4-(4'-dimethylaminophenyl)-1,2,4-triazoline-3,5-dione (DAPTAD), which rapidly and quantitatively reacts with the s-cis-diene structure of vitamin D metabolites. The derivatized samples were subjected to LC-MS/MS, ionized by electrospray ionization (positive-ion mode), and detected by selected reaction monitoring. The lower limits of quantification for 25(OH)D3, 3-epi-25(OH)D3, 25(OH)D2, and 24,25(OH)2D3 were 0.091, 0.020, 0.013, and 0.024 ng/mL, respectively. The accuracy values and the extraction recoveries for these four metabolites were satisfactory. Serum 25(OH)D levels determined by our LC-MS/MS were compared with those obtained by conventional radioimmunoassay (RIA) that cannot distinguish 25(OH)D3 and 25(OH)D2. The values obtained by the RIA method exhibited a mean bias of about 8.35 ng/mL, most likely as a result of cross reaction of the antibody with low-abundance metabolites, including 24,25(OH)2D3. Various preanalytical factors, such as long sample sitting prior to serum separation, repeated freeze-thaw cycles, and the presence of anticoagulants, had no significant effects on these determinations. This high-throughput LC-MS/MS simultaneous assay of the four vitamin D metabolites 25(OH)D3, 3-epi-25(OH)D3, 25(OH)D2, and 24,25(OH)2D3 required as little as 20 μL serum. This method will aid further understanding of low-abundance vitamin D metabolites, as well as the accurate determination of 25(OH)D3 and 25(OH)D2.