Abstract
Measured serum 25-hydroxyvitamin D concentrations vary depending on the type of assay used and the specific laboratory undertaking the analysis, impairing the accurate assessment of vitamin D status. We investigated differences in serum 25-hydroxyvitamin D concentrations measured at three laboratories (laboratories A and B using an assay based on liquid chromatography-tandem mass spectrometry and laboratory C using a DiaSorin Liaison assay), against a laboratory using an assay based on liquid chromatography-tandem mass spectrometry that is certified to the standard reference method developed by the National Institute of Standards and Technology and Ghent University (referred to as the ‘certified laboratory’). Separate aliquots from the same original serum sample for a subset of 50 participants from the Ausimmune Study were analysed at the four laboratories. Bland-Altman plots were used to visually check agreement between each laboratory against the certified laboratory. Compared with the certified laboratory, serum 25-hydroxyvitamin D concentrations were on average 12.4 nmol/L higher at laboratory A (95% limits of agreement: -17.8,42.6); 12.8 nmol/L higher at laboratory B (95% limits of agreement: 0.8,24.8); and 10.6 nmol/L lower at laboratory C (95% limits of agreement: -48.4,27.1). The prevalence of vitamin D deficiency (defined here as 25-hydroxyvitamin D <50 nmol/L) was 24%, 16%, 12% and 41% at the certified laboratory, and laboratories A, B, and C, respectively. Our results demonstrate considerable differences in the measurement of 25-hydroxyvitamin D concentrations compared with a certified laboratory, even between laboratories using assays based on liquid chromatography-tandem mass spectrometry, which is often considered the gold-standard assay. To ensure accurate and reliable measurement of serum 25-hydroxyvitamin D concentrations, all laboratories should use an accuracy-based quality assurance system and, ideally, comply with international standardisation efforts.
Highlights
Vitamin D status is assessed by measuring serum concentrations of total 25-hydroxyvitamin D (25(OH)D), which is the sum of two metabolites, 25(OH)D3 and 25(OH)D2
We investigate the differences in serum 25(OH)D concentrations measured at three laboratories compared with a laboratory using an LC-MS/ MS-based assay that is certified to the reference measurement procedure (RMP)
Accuracy-based quality assurance schemes such as D External Quality Assessment Scheme (DEQAS) and the Centers for Disease Control and Prevention (CDC) standardisation program provide metrics for the accuracy of the assay against the RMP developed by National Institute of Standards and Technology (NIST) and Ghent University
Summary
Vitamin D status is assessed by measuring serum concentrations of total 25-hydroxyvitamin D (25(OH)D), which is the sum of two metabolites, 25(OH)D3 and 25(OH)D2. Given the current interest in vitamin D status and its purported relationship with a wide range of health outcomes, the accurate and reliable assessment of serum 25(OH)D concentrations, in both research and clinical settings, is imperative. A high prevalence of vitamin D deficiency has been reported in populations worldwide based on measurement of serum 25(OH)D concentrations [4,5,6,7,8]. Deficient or insufficient vitamin D status may signify a need for lifestyle and/or dietary changes, including supplementation
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