Assay variation confounds the diagnosis of hypovitaminosis D: a call for standardization.

By N. Binkley, D. Krueger, C. S. Cowgill, L. Plum, E. Lake, K. Hansen, H. DeLuca, M. Drezner. Journal of Clinical Endocrinology and Metabolism, 2004

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Overview of general physiologic features and functions of vitamin D. (2004, result): “Unfortunately, commercially available assays for 25(OH)D3 yielded widely differing results (Figure 11) (60).”
Vitamin D deficiency: a worldwide problem with health consequences. (2008, background): “These include various radioimmunoassays (98) and what is now considered to be the gold standard: liquid chromatography–tandem mass spectroscopy (14).”
Serum 25-hydroxyvitamin D status of the US population: 1988–1994 versus 2000–2004 (2008, positive): “Binkley et al (16) found that differences in mean serum 25(OH)D concentrations between different assay types could be as much as 2-fold when the same set of blood samples were measured.”