Analytical performance of an automated LC-MS/MS analyzer for determination of vitamin D concentration in blood plasma and serum

Abstract Objectives Measurement of the concentrations of vitamin D metabolites is clinically important because abnormal levels are associated with perturbations in calcium and phosphorus homeostasis. The objective of this study was to evaluate analytical performance of the Ionify ® Vitamin D iPack that measures 25-hydroxyvitamin D 25(OH)D and 24,25-dihydroxyvitamin D 24,25(OH) 2 D on the Cobas ® i 601 analyzer, an automated liquid chromatography and tandem mass spectrometry (LC-MS/MS) analyzer. Methods Repeatability and reproducibility were assessed in three laboratories (nine samples, five replicates per day for five days). Accuracy was evaluated using three to four external quality assessment (EQA) samples in six laboratories. Finally, the Cobas i 601 method was compared to a validated in-house LC-MS/MS laboratory-developed test (LDT) using weighted Deming regression. Results Across the three sites and reagent lots, coefficients of variation (%CV) for 25(OH)D and 24,25(OH) 2 D levels ranged from 2.7 % to 4.8 % and 2.3 % to 3.5 %, respectively. Accuracy (median recovery for all samples) ranged from 89 % to 110 % of the expected value, and from 92 % to 109 % of the reference method, across six laboratories. 25(OH)D and 24,25(OH) 2 D concentrations measured on the Cobas i 601 system were highly correlated (r value between 0.984 and 0.995, slope 0.913 to 1.027) with those from the validated LDT. Conclusions The Ionify Vitamin D iPack on the Cobas i 601 analyzer is a precise, accurate and reliable method for 25(OH)D and 24,25(OH) 2 D measurement and can support clinical management of vitamin D deficiency and excess.