Current status of clinical 25-hydroxyvitamin D measurement: An assessment of between-laboratory agreement.

Clin Chim Acta. 2010 Aug 13.
Binkley N, Krueger DC, Morgan S, Wiebe D.
University of Wisconsin Osteoporosis Clinical Center and Research Program.

BACKGROUND: Historically, methodological differences and lack of standardization led to between-laboratory variability in 25(OH)D results. Recent observations raised concern about persisting variability. This quality assurance exercise investigated 25(OH)D result comparability between laboratories.

METHODS: Serum pools (n=25) were prepared to contain endogenous 25(OH)D(2) and 25(OH)D(3) at 25(OH)D concentrations from ~12-150nmol/L (5-60ng/mL).
Aliquots were sent to 8 laboratories utilizing various 25(OH)D assay methods including
high performance liquid chromatography with ultraviolet detection (LC-UV),
LC with tandem mass spectroscopy detection (LC-MS/MS) or an
automated immunoassay (Diasorin Liaison).

The LC-UV results were selected as a referent to which all others were compared using linear regression and Bland-Altman analysis.

RESULTS: Good correlation (R(2)=0.87 to 0.97) was observed for all laboratories. Modest systematic bias was observed for some laboratories ranging from a
positive mean bias of 10.5nmol/L (4.2ng/mL) to a
negative mean bias of 3.5nmol/L (1.4ng/mL).
For the laboratory with the greatest bias, 22/25 results were numerically higher (mean +15.7%) than LC-UV results. For Liaison, the primary error was likely random, whereas the major LC-MS/MS assay error source were biases likely due to calibration issues.

CONCLUSIONS: Modest inter-laboratory variability persists in serum 25(OH)D measurement. The National Institute of Standards & Technology 25(OH)D Standard Reference and calibration materials will further improve between-laboratory agreement for chromatography-based assays. PMID: 20713030