Effect of 25(OH) vitamin D reference method procedure (RMP) alignment on clinical measurements obtained with the IDS-iSYS chemiluminescent-based automated analyzer

The Vitamin D Standardization Program (VDSP) has identified ID-LC/MS/MS as the reference method procedure (RMP) for 25(OH) vitamin D and NIST Standard SRM2972 as the standard reference material (SRM). As manufacturers align their products to the RMP and NIST standard, a concern is that results obtained in aligned assays will be divergent from those obtained with pre-alignment assays. The Immunodiagnostic Systems Ltd.; chemiluminescent, 25(OH) vitamin D iSYS platform assay, was recently harmonized to the RMP. To determine the impact of standardization on results obtained with iSYS reagents, 119 single donor serum samples from eight different disease categories were analyzed in four non-standardized and two standardized iSYS assays. There were strong correlations between the four non-standardized and two standardized assays with Spearman's rank r values between 0.975 and 0.961 and four of the eight r values were >0.97. R² values for the eight best-fit linear regression equations ranging between 0.947 and 0.916. None of the slopes were found to be significantly different from one another. Bland-Altman plots showed that the bias was comparable when each of the four non-standardized assays was compared to either of the standardized assays. When the data were segregated in values between 6 and 49 ng/mL (15-122 nmol/L) or between 50 and 100 ng/mL (125-250 nmol/L) significant associations remained between results obtained with non-standardized and standardized calibrators regardless of the absolute value. When five recent DEQAS unknowns were analyzed in one non-standardized and one standardized assay the mean percent difference from the NIST target in values obtained using standardized vs. non-standardized calibrators were not significantly different. Finally, strong and statistically significant associations between the results were obtained using non-standardized and standardized assays for six of eight clinical conditions. The only exceptions were hypocalcemia and breast cancer, which likely reflect the small sample sizes for each of these diseases. These initial data provide confidence that the move to a NIST standardized assay will have little impact on results obtained with the iSYS platform. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.