Vitamin D standardisation

Abstract

Vitamin D has been implicated in a host of conditions, including certain cancers, type 1 diabetes, multiple sclerosis, tuberculosis, Alzheimer's disease and psoriasis. Some laboratories report test requests increasing by over 100% per year. Vitamin D measurement is performed using a number of technologies including immunoassay, high performance liquid chroma-tography (HPLC) and LC-MS/MS. The UK-based Vitamin D External Quality Assessment Scheme (DEQAS), the largest vitamin D proficiency testing program with over 600 registered participants, has observed extensive inter-laboratory variation. A 2008 review found that the annual mean inter-laboratory imprecision has generally declined from near 30% in the mid 1990s to under 20% in 2007, with sporadic increases as new methods were introduced and problems with certain assays were identified. Significantly, DEQAS analysis revealed that between April 2007 and January 2008, the LCMS method produced a mean positive bias of >10%. NIST has been developing serum and solvent-based standard reference materials for 25-OH-D (SRM 972 and SRM 2972, respectively). In theory, the SRMs will provide both immunoassay users (in the case of SRM 972) and LC-MS users (in the case of SRM 2972) a means to calibrate their assays, reducing the inter-laboratory coefficients of variation (CVs) for each method. One study found that harmonisation to a common calibrator reduced average between laboratory variation to 9% from 30%.