25-Hydroxyvitamin D Assays: The Quest For Accuracy

[...] immunoassay kits are subject to performance changes over time, perhaps due to the reformulation of standards or reagents (a change of antibody being an extreme example of the latter). Nonimmunoassay methods such as LC-MS/MS should be less vulnerable to long-term performance change, problems with calibration notwithstanding. [...] a new body of data is collected with assays calibrated against an accepted RMP, 25-OH-D results should be interpreted with due regard to methodologic differences and without recourse to "fudge factors," which are inherently unreliable.