Comparison of two LC-MS/MS methods for the quantification of 24,25-dihydroxyvitamin D3 in patients and external quality assurance samples.

Objectives: In-house developed liquid-chromatography mass spectrometry (LC-MS/MS) methods are used more and more frequently for the simultaneous quantification of vitamin D metabolites. Among these, 24,25-dihydroxyvitamin D3 (24,25(OH)D) is of clinical interest. This study assessed the agreement of this metabolite in two validated in-house LC-MS/MS methods.

Methods: 24,25(OH)D was measured in 20 samples from the vitamin D external quality assurance (DEQAS) program and in a mixed cohort of hospital patients samples (n=195) with the LC-MS/MS method at the Medical University of Graz (LC-MS/MS 1) and at the University of Liège (LC-MS/MS 2).

Results: In DEQAS samples, 24,25(OH)D results with LC-MS/MS 1 had a proportional bias of 1.0% and a negative systemic difference of -0.05%. LC-MS/MS 2 also showed a proportional bias of 1.0% and the negative systemic bias was -0.22%. Comparing the EQA samples with both methods, no systemic bias was found (0.0%) and the slope was 1%. The mean difference of 195 serum sample measurements between the two LC-MS/MS methods was minimal (-0.2%). Both LC-MS/MS methods showed a constant bias of 0.31 nmol/L and a positive proportional bias of 0.90%, respectively.

Conclusions: This study is the first to assess the comparability of 24,25(OH)D concentrations in a mixed cohort of hospitalized patients with two fully validated in-house LC-MS/MS methods. Despite different sample preparation, chromatographic separation and ionization, both methods showed high precision measurements of 24,25(OH)D. Furthermore, we demonstrate the improvement of accuracy and precision measurements of 24,25(OH)D in serum samples and in the DEQAS program.