Current status of clinical 25-hydroxyvitamin D measurement: an assessment of between-laboratory agreement.
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 to 150 nmol/l (5-60 ng/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 10.5 nmol/l (4.2 ng/ml) to a negative mean bias of 3.5 nmol/l (1.4 ng/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 was biases likely due to calibration issues.


CONCLUSIONS
Modest inter-laboratory variability persists in serum 25(OH)D measurement. The National Institute of Standards and Technology 25(OH)D Standard Reference and calibration materials will further improve between-laboratory agreement for chromatography-based assays.
Binkley
Neil
N
University of Wisconsin Osteoporosis Clinical Center and Research Program, Madison, WI 53705, United States. nbinkley@wisc.edu
Krueger
Diane C
DC
Morgan
Sarah
S
Wiebe
Donald
D
D001774:Blood Chemical Analysis / Q000379:methods / Q000592:standards*; D006077:Government Agencies / Q000592:standards; D006801:Humans; D007753:Laboratories / Q000592:standards*; D012015:Reference Standards; D015203:Reproducibility of Results; D014481:United States; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000032:analysis / Q000097:blood
D014807:Vitamin D; C104450:25-hydroxyvitamin D