Inter-laboratory harmonization of microsphere immunoassays for SARS-CoV-2 antibody detection in contrived dried blood spots and oral fluids

Dried blood spots (DBS) and oral fluids (OF) are easily attainable biospecimen types that enabled population-scale antibody monitoring for SARS-CoV-2 exposure and vaccination. However, the degree to which the two different biospecimen types can be used interchangeably remains unclear. To address this question, we generated contrived DBS (cDBS) and contrived OF (cOF) from serum panels of SARS-CoV-2-infected, vaccinated, and uninfected individuals. The contrived samples were evaluated using SARS-CoV-2 multiplexed microsphere immunoassays (MIAs) at two different institutions. Intra-laboratory tests revealed near-perfect agreement between cDBS and cOF for N and S antigens, as evidenced by κ = 0.97–1 and 98%–100% agreement. Interlaboratory comparisons were equally robust for both N (κ = 0.94–0.96; 97.5%–98 % agreement) and S (κ = 0.98–1.0; 99.0%–100%). Furthermore, assays were transferred between labs, including methods and reagents, and a subset of cDBS and cOF samples (n = 52) were tested. Qualitative concordance remained high (κ = 0.94–1.0; 97.5%–100% agreement), confirming that the integrity of the assays is retained upon transfer. In summary, our results provide evidence that DBS and OF can be used interchangeably across laboratories and institutions for the qualitative assessment of SARS-CoV-2 antibody determinations.