Evaluation of the Analytical and Clinical Concordance of 25-Hydroxyvitamin D Levels in Dried Blood Spots, Dried Serum Spots, and Serum as Potential Biorepository Specimens.

CONTEXT: The promising perspective of storing dried blood spot (DBS) for evaluating 25-hydroxyvitamin D (25OHD) levels is increasingly being realized. While strong correlations have been demonstrated between 25OHD levels measured in DBS and in systemic serum samples in earlier works, the clinical concordance of the assay results has not been evaluated. Moreover, the utility of dried serum spot (DSS), a highly suitable matrix for sample archiving, has not been investigated in this respect.

METHODS: 25-hydroxycholecalciferol and 25-hydroxyergocalciferol levels were established selectively in DBS (n = 73) and DSS (n = 67) specimens obtained from deidentified whole blood and serum using a previously validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. In addition, total 25OHD levels were determined in the serum samples using the LIAISON 25OH Total Vitamin D Assay (LIA, n = 73). The analytical and clinical performance of the three approaches was compared pairwise.

RESULTS: Deming regression, Bland-Altman analysis, and concordance correlation coefficients consistently demonstrated the lack of analytical equivalence among the three result sets. The overall percentage agreement of the clinical classifications (hypovitaminosis or euvitaminosis) was moderate (67.1%-83.6%). The delivery of positive cases was decreasing significantly in the order LIA>DSS>DBS (p < 0.05).

CONCLUSIONS: The approaches tested did not deliver equivalent outputs either in an analytical or a clinical context. Therefore, specific reference ranges must be established for each matrix to avoid false clinical evaluation. 25OHD can be quantified when assay results are scaled by a factor of 1.60-1.67. Considering the convenience and efficiency of the storage and processing of DSS, along with the difficulties of quantifying 25OHD in real-life DBS samples accurately, DSS is proposed as an alternative for the long-term archiving of specimens.