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. (C) 2010 Elsevier B.V. All rights reserved.