Serum-equivalency comparison, detection, and quantification of Group B <i>Streptococcus</i> anti-capsular polysaccharide antibodies from dried blood spots

A standardized multiplex immunoassay (MIA) to quantify group B <i>Streptococcus</i> (GBS) anti-capsular polysaccharide (CPS) IgG serum concentrations was adopted by the Group B Streptococcal Assay Standardization (GASTON) consortium as a standardized serological assay with the most immediate applications for facilitating the licensure of GBS vaccines. However, dried blood spot (DBS) samples offer advantages for immunological studies, including cost-effectiveness, ease of transport, and storage. To determine suitability of DBS as an alternative sample matrix to serum in MIA, a contrived GBS seropositive panel, including matched DBS and serum samples, was prepared using established methods. The calculated geometric mean titers of GBS anti-CPS IgG values by individual serotype were compared using a paired t-test to establish serum equivalency. Geometric mean values for the matched panel were assessed via Deming regression for precision, accuracy, and concordance correlation coefficient (CCC). The initial acceptance criterion was set at 0.95 for CCC. Two additional criteria based on confidence intervals of CCC, slope, and intercept were used to determine the necessity of a serotype-specific conversion factor. The paired t-test <i>p</i>-values were &gt;.05 for serum equivalency. For sample matrix concordance, CCC values were &gt;0.95 and met correlation criteria for all serotypes. Conversion factors were applied to four serotypes (II, III, IV, and V) that did not meet the criteria for slope, intercept, or both. This demonstration of equivalency between DBS and serum supports the hypothesis that DBS is a suitable testing matrix from which to elucidate anti-CPS IgG concentrations in seroepidemiological and vaccine evaluation studies.