Background: Previous studies have shown that the measurement of vitamin D and its derivatives, especially its active metabolite 1α, 25-dihydroxy-vitamin D [1,25(OH)2D], is highly complex and prone to analytical error. We have evaluated a new immunological method for detecting and quantifying of 1,25(OH)2D. This assay is fully automated, sensitive and uses a specific recombinant fusion protein for capturing of 1,25(OH)2D. The assay was originally developed by DiaSorin for the immunoassay analyzer LIAISON XL.
Methods: Performance data of this assay were determined including intra- and inter-assay precision, recovery, linearity, and limit of detection of the DiaSorin 1,25(OH)2D immunoassay on the LIAISON XL analyzer. Respective data were compared from two different liquid chromatography tandem mass spectrometry (LC-MS/MS) assays and a common radioimmunoassay (RIA) using clinical samples taken from patients suffering from vitamin D deficiency, chronic renal failure, biliary atresia, hyperparathyroidism, vitamin D-dependent rickets or sarcoidosis, as well as from pregnant women and high-level athletes.
Results: The performance evaluation of 1,25(OH)2D resulted in an intra-assay and total imprecision correlation variant between 1.4% and 5.2% and 3.8%-7.1% with the new immunoassay and 3.5%-5.8% or 3.8%-7.5% with the LC-MS/MS method, respectively. Limits of detection and quantification of the immunoassay were 0.7 ng/L and 5.0 ng/L for the LIAISON XL immunoassay and 1.8 ng/L and 5.4 ng/L for the LC-MS/MS assay, respectively. Pearson's coefficients of correlation were 0.998 and 0.952 for method comparison to different established LC-MS/MS methods. Linear regression according to Passing and Bablok showed larger deviations to the RIA (slopes 0.64-0.97, coefficients of correlation 0.822-0.823).
Conclusions: The DiaSorin LIAISON XL 1,25(OH)2D immunoassay appears to have improved comparability to LC-MS/MS with low imprecision and limits of detection. The assay time of 65 min, the small sample volume required (75 μL) and the throughput of 90 tests/hour without manually handling time for extraction and purification procedures is superior to the LC-MS/MS method.