Deuterated stable isotope (d3-25-hydroxyvitamin D) is added to a 0.1-mL serum sample as internal standard. 25-Hydroxyvitamin D2 (25-OH-VitD2), 25-hydroxyvitamin D3 (25-OH-VitD3), and the internal standard are extracted with acetone and ethyl acetate. The extracts are then derivatized before being analyzed by liquid chromatography-tandem mass spectrometry using multiple reaction monitoring. 25-OH-VitD2 and 25-OH-VitD3 are quantified and reported individually and as a sum with a clinical reference range attached to the sum. For children <1 year of age, a modified assay is performed to identify and eliminate interference due to C-3 epimers of 25-OH-VitD2 and 25-OH-VitD3.(Zimmer D, Pickard V, Czembor W, Muller C: Comparison of turbulent-flow chromatography with automated solid-phase extraction in 96-well plates and liquid-liquid extraction used as serum sample preparation techniques for liquid chromatography-tandem mass spectrometry. J Chromatogr A 1999 Aug 27;854[1-2]:23-35)
Monday through Friday; Continuous until 2:30 p.m.
Specimens on patients who are <1 year old are only performed on Tuesday and Friday.