Vitamin D testing is increasing worldwide. Recently several diagnostic manufacturers including Abbott and Siemens have launched automated 25-hydroxy vitamin D (25OH-D) immunoassays. Furthermore, preexisting assays from DiaSorin and Roche have recently been modified. We compared the performance of 5 automated immunoassays, an RIA and 2 liquid chromatography–tandem mass spectrometry (LC-MS/MS) methods.

Aliquots of 170 randomly selected patient samples were prepared and 25OH-D was measured by 2 LC-MS/MS methods, an RIA (DiaSorin), and automated immunoassays from Abbott (Architect), DiaSorin (LIAISON), IDS (ISYS), Roche (E170, monoclonal 25OH-D₃ assay), and Siemens (Centaur). Within-run and between-run imprecision were evaluated by measurement of 5 replicates of 2 serum pools on 5 consecutive days.

The LC-MS/MS methods agreed, with a concordance correlation coefficient (CCC) of 0.99 and bias of 0.56 μg/L (1.4 nmol/L). The RIA assay showed a performance comparable to LC-MS/MS, with a CCC of 0.97 and a mean bias of 1.1 μg/L (2.7 nmo/L). All immunoassays measured total 25OH-D (including D₃ and D₂), with the exception of the Roche assay (D₃ only). Among the immunoassays detecting total 25OH-D, the CCCs varied between 0.85 (Abbott) to 0.95 (LIAISON). The mean bias ranged between 0.2 μg/L (0.5 nmol/L) (LIAISON) and 4.56 μg/L (11.4 nmol/L) (Abbott). The Roche 25OH-D₃ assay demonstrated small mean bias [−2.7 μg/L (−6.7 nmol/L)] [−2.7 μg/L (−6.7 nmol/L)] but a low CCC of just 0.66. Most assays demonstrated good intra- and interassay precision, with CV <10%.

Automated immunoassays demonstrated variable performance and not all tests met our minimum performance goals. It is important that laboratories be aware of the limitations of their assay.