Evaluation of the new restandardized 25-hydroxyvitamin D assay on the iSYS platform

Background: 25-hydroxyvitamin D [25(OH) D] is the most reliable biomarker of vitamin D status, but until now 25(OH) D assays have suffered from inter-laboratory and inter-assay discrepancies. In the setting of the international Vitamin D Standardization Program, Immunodiagnostic Systems (IDS) recently reformulated and restandardized the 25(OH) D immunoassay available on the automated iSYS platform. In the present study, we evaluated this new generation of the 25(OH) D immunoassay (IS-2500). Methods: Repeatability and within-laboratory imprecision were verified according to the Clinical and Laboratory Standards Institute EP15-A3. Results from the sera of 63 patients were compared with those obtained with the previous iSYS method (IS-2700S) using Passing-Bablok and Bland-Altman analysis. The prevalence and bias-adjusted kappa (PABAK) coefficient was calculated to assess the agreement of vitamin D status provided by the two iSYS immunoassays. Fourteen Vitamin D External Quality Assessment Scheme (DEQAS) samples were used to evaluate inaccuracy. Results: Using the EP15-A3 protocol, repeatability and within-laboratory imprecision obtained with the new iSYS method were lower than 6% and 8%, respectively. These results are consistent with the manufacturer's claims. In more adverse conditions (50 measurements over 15 days with multiple calibrations), the within-laboratory imprecision was 14.8% (39 nmol/L) and 7.7% (155 nmol/L). 25(OH) D concentrations measured with the new assay showed a strong correlation with those provided by the previous version (r = 0.969, p < 0.0001). The Passing-Bablok regression equation was as follows: new assay = 1.079 x (previous assay) - 3.6 nmol/L. The PABAK coefficient of 0.810 reflected almost perfect agreement between the two immunoassays to classify patients according to their vitamin D status (85.7% of agreement). Using DEQAS samples, the mean inaccuracy bias was lower than 5% when the new iSYS method was compared with LC-MS/MS methods and the NIST reference measurement procedure. Conclusion: The new generation of the iSYS immunoassay evaluated in this study meets requirements for routinely measuring 25(OH) D levels in clinical laboratories.