Evaluation of a new automated assay for the measurement of circulating 1,25-dihydroxyvitamin D levels in daily practice.

Abstract Background and objective 1,25-dihydroxyvitamin D [1,25(OH) 2 D] is the most important vitamin D metabolite in regulating calcium and phosphorus metabolism and bone resorption. It is measured in plasma for diagnosis and management of patients with disorders influencing vitamin D metabolism. The most common methods for 1,25(OH) 2 D quantification are competitive isotopic I 125 immunoassays. The purpose of this work is to compare the IDS isotopic immunoassay (IDS RIA) and the nonisotopic iSYS immunoassay (IDS iSYS) for 1,25(OH) 2 D measurement in human serum and in control samples obtained from the Vitamin D External Quality Assessment Scheme (DEQAS). Methods 1,25(OH) 2 D concentrations in 180 serum samples (87 females and 93 males) and in 25 samples from the DEQAS were assayed using IDS RIA and IDS iSYS methods. Both assays were performed after delipidation and immunoextraction steps. Measurement range was 9.0–348.0 pmol/L and 15.6–504.0 pmol/L for IDS RIA and IDS iSYS assay, respectively. ‘Normal adult’ reference ranges of 1,25(OH) 2 D were 43–168 pmol/L for IDS RIA and 63–228 pmol/L for IDS iSYS. Results The equation of the Deming regression analysis demonstrated that IDS iSYS tended to give lower 1,25(OH) 2 D concentrations by a mean of 20% over the tested concentration range when compared to IDS RIA. Both assays would easily meet the DEQAS goal of 80% of survey samples within 30% of the All-Laboratory Trimmed Mean “ALTM”. Results of 1,25(OH) 2 D obtained using IDS iSYS in samples distributed by DEQAS were closely related to those by LC–MS/MS method. Conclusions The concentrations of 1,25(OH) 2 D measured by the IDS iSYS tended to be lower than those of IDS RIA in patients, but quite comparable to those of LC–MS/MS and ALTM in DEQAS samples.