25-Hydroxyvitamin D testing: challenging the performance of current automated immunoassays

Background: Clinical laboratories require accurate and precise 25-hydroxyvitamin D (25-OHD) immunoassays to allow comparison of patient results with published decision limits. However, some variation in performance has been found with the previous generation of automated 25-OHD immunoassays. This study assessed the performance of four recently released automated 25-OHD immunoassays against a liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay. Methods: A total of 983 samples from apparently healthy adults, plus 253 samples chosen to challenge the performance of the assays, were analyzed by the latest generation of immunoassays from Abbott, DiaSorin, Roche and Siemens. LC-MS/MS analysis was performed on a random subset of 264 samples. The precision of the immunoassays was assessed over 5 days with samples ranging between 3.0 and 370 nmol/L in concentration. Results: Immunoassays showed significant differences in precision at 25-OHD concentrations of 3.0-86.5 nmol/L but all showed acceptable precision at higher concentrations. The DiaSorin assay agreed with LC-MS/MS across the measuring range of samples tested (7.7-425 nmol/L). The other assays showed generally good performance, but had some limitations when their performance was challenged with samples with low and high 25-OHD concentrations, heterophilic antibodies or high 25-OHD2 concentrations. The C3-epimer of 25-OHD was identified in 40.4% of healthy adults tested and was a source of analytical variance in immunoassays. Conclusions: The latest generation of 25-OHD immunoassays has improved performance compared to previous assays. However, some immunoassays can still give discrepant results and this is most apparent when immunoassays are evaluated with a range of samples that challenge their analytical performance.