Routine isotope-dilution liquid chromatography-tandem mass spectrometry assay for simultaneous measurement of the 25-hydroxy metabolites of vitamins D2 and D3.
BACKGROUND
Measurement of 25-hydroxyvitamin D2 and D3 (25-OH D2 and D3) is essential for investigating vitamin D deficiency. Competitive binding techniques are unable to distinguish between the 2 metabolites and suffer from interference from other hydroxy metabolites of vitamin D.


METHODS
We used isotope-dilution liquid chromatography-tandem mass spectrometry (ID-LC-MS/MS) for routine determination of 25-OH D2 and D3 with a stable-isotope-labeled internal standard (IS). Serum samples (100 microL) were denatured with methanol-propanol containing IS, vortex-mixed, extracted into hexane, and dried under nitrogen. The reconstituted extract was chromatographed on a BDS C8 HPLC column, and the metabolites and IS were detected by electrospray ionization MS/MS in multiple-reaction monitoring mode.


RESULTS
25-OH D2 and D3 and the IS nearly coeluted, whereas 1alpha-hydroxyvitamin D3 was separated; total run time was 8 min. The interassay CVs for 25-OH D2 were 9.5% and 8.4% at 52 and 76 nmol/L, respectively, and for 25-OH D3 were 5.1% and 5.6% at 55 and 87 nmol/L, respectively. The detection limit of the present method was <4 nmol/L for both metabolites. Method comparison with a commercial RIA measuring total 25-hydroxyvitamin D showed good correlation: y=0.97x - 2.7 nmol/L (r=0.91). The analytical system can assay 100 samples in 12.5 h.


CONCLUSIONS
This simple robust interference-free LC-MS/MS assay is suitable for routine measurement of the 25-hydroxy metabolites of vitamins D2 and D3 in human serum. The assay has been in use for 9 months and has been used to assay more than 6000 routine samples.
Maunsell
Zoë
Z
Department of Clinical Biochemistry, Northwick Park Hospital, North West London Hospitals NHS Trust, Harrow, United Kingdom.
Wright
Dennis J
DJ
Rainbow
Sandra J
SJ
D015652:25-Hydroxyvitamin D 2 / Q000097:blood*; D002112:Calcifediol / Q000097:blood*; D002851:Chromatography, High Pressure Liquid; D003903:Deuterium; D006801:Humans; D006887:Hydroxycholecalciferols / Q000097:blood; D007201:Indicator Dilution Techniques; D011863:Radioimmunoassay; D021241:Spectrometry, Mass, Electrospray Ionization
D006887:Hydroxycholecalciferols; D015652:25-Hydroxyvitamin D 2; D003903:Deuterium; D002112:Calcifediol; C008088:alfacalcidol