Surveillance evaluation of the standardization of assay values for serum total 25-hydroxyvitamin D concentration in Japan

Background: To assess the vitamin D nutritional status, serum total 25-hydroxyvitamin D (25(OH)D) concentration is measured. We used six automated 25(OH)D immunoassays (AIAs) available in Japan and certified by the Vitamin D Standardization Program (VDSP) at the U.S. Center for Disease Control and Prevention to assess the concordance of the assay results. Methods: Serum total 25(OH)D concentrations in SRM 972a and 20 serum samples from patients were determined using three liquid chromatography-tandem mass spectrometry (LC-MS/MS) and six AIAs (pilot study), and an additional 110 serum samples were assessed by the six AIAs (surveillance study). The assay bias from the results of LC-MS/MS by Chiba University or consensus values (i.e. average of six AIAs) was estimated using the procedure described in CLSI document EP09-A3. Results: LC-MS/MS at Chiba University could completely separate 25(OH)D2, 25(OH)D3 and 3-epi-25(OH)D3, and the observed values including total 25(OH)D in SRM 972a were all within ±1·SD of the assigned values. All AIAs produced results greater than ±3·SD. In the pilot study, four of the six AIAs had an average percentage bias, as estimated by confidence interval (CI), larger than ±5% (acceptance criterion in CLSI); the bias converged from −6.5% to 3.2% after adjustment by LC-MS/MS. In the surveillance study, 25(OH)D concentrations in AIAs all adjusted to LC-MS/MS converged within ±5% from consensus values. However, some AIAs showed negative or positive bias from the consensus values. Conclusions: Current AIAs in Japan continue to lack standardization. Manufacturers should implement quality assurance strategies so that their values more closely align to those of standard reference material 972a.