a Anti-RBD recombinant human antibody (IgG and IgM) was added to whole blood from two seronegative donors, then absorbed to microsamplers and remaining blood was spun down to isolate serum and analyzed on full spike ectodomain trimer (spike) or receptor-binding domain (RBD) ELISA. Data are mean ± SEM, n = 3, red = microsampler eluate, gray = matched serum. b Direct comparison of absorbance of range of recombinant antibody concentration in serum (y axis) versus microsampler (x axis) blood samples. c Sigmoidal four-parameter logistic (4PL) curve fitting to recombinant antibody dilution series, 95% confidence intervals shown shaded around fit curve, n = 12 replicates per data point, red = Spike IgG, orange = RBD IgG, blue = Spike IgM, purple = RBD IgM. d Quantification of IgG levels in a sample high-incidence population. e Upper limit of quantification at 1:400 dilution of serum into ELISA (1:10 dilution of microsampler eluate), n = 68. Source data are provided as a Source Data file.