Objectives: As (a lack of) vitamin D has been linked to a wide variety of chronic diseases, there is growing interest in generating robust epidemiological data. Consequently, the need for large-scale biological sample collection has increased. In this context, dried blood spot (DBS) microsampling offers a minimally invasive alternative to venous sampling. However, to allow interpretation of DBS-based 25-hydroxyvitamin D (25-(OH)D) results, the set-up of a clinical validation study is essential to assess the agreement with the reference matrix, plasma.
Methods: Venous plasma and whole blood, venous DBS (vDBS) and capillary DBS (cDBS) were collected from 44 healthy volunteers to evaluate the agreement between the different sample types for 25-(OH)D quantification. To transform cDBS-based results to plasma concentrations, a hematocrit (Hct)-dependent conversion factor was applied and evaluated using four different Hct determination approaches. All samples were analysed using previously described validated LC-MS/MS methods.
Results: No clinically relevant methodological (vDBS vs. whole blood) or sampling-site related (cDBS vs. vDBS) issues were observed. Following Hct-dependent conversion, good agreement between the cDBS-derived and actually measured plasma results was obtained, as 90 % of the results lay within 20 % of the plasma result, independent of the Hct approach used. Additionally, weighted Cohen's kappa values of 0.83-0.85 were obtained across the different Hct approaches, indicating substantial to almost perfect agreement in vitamin D status classification.
Conclusions: Following Hct-dependent conversion, cDBS can be used as a reliable and practical alternative matrix to plasma for large-scale monitoring of vitamin D status in epidemiological and public health contexts.
Keywords: 25-hydroxyvitamin D; LC-MS/MS; clinical validation; dried blood spots; hematocrit.
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