Analytical considerations for the biochemical assessment of vitamin D status.

The most widely used and clinically accepted biochemical marker for assessing vitamin D status is the total serum 25-hydroxyvitamin D [25(OH)D] concentration. Despite the analysis of 25(OH)D dating back to the early 1970s, modern analytical techniques still exhibit significant interassay variability due to varying concentrations of other related vitamin D metabolites and sample-to-sample matrix differences. It is important for clinicians requesting 25(OH)D analyses to understand these issues and limitations, and where necessary to confront laboratories for details of analytical methods used. The availability of reference measurement procedures for 25(OH)D based on liquid chromatography and tandem mass spectrometry, whilst not intended for routine clinical sample analysis, should be utilized to improve assay harmonization and reduce interlaboratory variability. Laboratories should also be forthcoming with details of subscriptions to external quality assessment schemes and assay traceability. As well as discussing the reasons for ongoing assay variability for 25(OH)D, this short review will also briefly discuss other assays related to the assessment of vitamin D status, including parathyroid hormone, 24,25-dihydroxyvitamin D, 1,25-dihydroxyvitamin D and vitamin D binding proteins.