Evaluation of 25-hydroxy vitamin D assay on the immunodiagnostic systems iSYS analyser

Background: We evaluated the recently released chemiluminescence assay for 25-hydroxy vitamin D (25-OHD) on the Immunodiagnostic Systems iSYS (IDS-iSYS) automated analyser. Methods: The IDS-iSYS comparison was performed using patient samples previously measured for 25-OHD by a validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) method (n = 119) and an IDS enzyme immunoassy (IDS-EIA) method (n = 64). Limit of detection and limit of quantification were determined from a precision profile. Imprecision was assessed using quality control material and pooled serum. External QAP material (Vitamin D External Quality Assessment Scheme, UK) was analysed to establish inaccuracy. Linearity was assessed by two dilution studies. Cross-reactivity was determined by three serial dilution studies of patient samples with known 25-OHD2 concentrations. Results: The IDS-iSYS correlated well with both established methods (iSYS = 1.03LC-MS/MS - 6.53, R-2 = 0.82 and iSYS = 1.07IDS-EIA - 1.61, R-2 = 0.86). Imprecision of the iSYS assay for IDS control material was 13.4% at 32 nmol/L, 10% at 78 nmol/L, 9.4% at 161 nmol/L, and for the pooled material 9.3% at 72 nmol/L and 5.6% at 158 nmol/L. The evaluation found the assay to be highly accurate (IDS-iSYS = 0.93ALTM + 3.79, R-2 = 0.94) and linear (obs(1) = 0.93exp(1) - 5.05, R-2 = 0.99 (P = 0.256); and obs(2) = 0.97exp(2) + 6.07, R-2 = 0.97 (P = 0.654); ALTM, all-laboratory trimmed mean). Cross-reactivity studies demonstrated no significant difference to the calculated total 25-OHD as measured by LC-MS/MS. Conclusions: Even though the imprecision of the iSYS was found to be greater than that of the LC-MS/MS and EIA methods, the performance characteristics of the IDS-iSYS 25-OHD assay are suitable for routine diagnostic purposes on a high throughput automated analyser.