Accuracy of 6 Routine 25-Hydroxyvitamin D Assays: Influence of Vitamin D Binding Protein Concentration

BACKGROUND: Recent recognition of its broad pathophysiological importance has triggered an increased interest in 25-hydroxyvitamin D [25(OH) D]. By consequence, throughput in 25(OH) D testing has become an issue for clinical laboratories, and several automated assays for measurement of 25(OH) D are now available. The aim of this study was to test the accuracy and robustness of these assays by comparing their results to those of an isotope dilution/online solid-phase extraction liquid chromatography/tandem mass spectrometry (ID-XLC-MS/MS) method. We put specific focus on the influence of vitamin D-binding protein (DBP) by using samples with various concentrations of DBP. METHODS: We used 5 automated assays (Architect, Centaur, iSYS, Liaison, and Elecsys), 1 RIA (Diasorin) preceded by extraction, and an ID-XLC-MS/MS method to measure 25(OH) D concentrations in plasma samples of 51 healthy individuals, 52 pregnant women, 50 hemodialysis patients, and 50 intensive care patients. Using ELISA, we also measured DBP concentrations in these samples. RESULTS: Most of the examined 25(OH) D assays showed significant deviations in 25(OH) D concentrations from those of the ID-XLC-MS/MS method. As expected, DBP concentrations were higher in samples of pregnant women and lower in samples of IC patients compared to healthy controls. In 4 of the 5 fully automated 25(OH) D assays, we observed an inverse relationship between DBP concentrations and deviations from the ID-XLC-MS/MS results. CONCLUSIONS: 25(OH) D measurements performed with most immunoassays suffer from inaccuracies that are DBP concentration dependent. Therefore, when interpreting results of 25(OH) D measurements, careful consideration of the measurement method is necessary. (C) 2011 American Association for Clinical Chemistry