Fig. 3: Two-bead strategy to detect specific antibodies in low volumes of plasma.

a, A schematic representation of the two-bead strategy for detection of antibodies in low volumes of plasma. b, A schematic representation of the baseline and flowthrough. The beads bound by anti-IgG magnetic beads are retained in the column, and the unbound beads are collected in the flow through. c, A dot plot of the number of barcoded beads in the baseline versus number in the flowthrough for 20 serum samples collected before the COVID-19 pandemic (green) and 20 samples from patients who had tested positive for COVID-19 (red). d, A comparison of ELISA (y axis) versus the two-bead assay (x axis) in the 20 COVID-19-positive samples shown in c. The two-bead assay result is shown as the ratio of the number of beads in the flowthrough to the number of beads in the baseline. Note that the two samples in c that behave as prepandemic samples also show low ELISA values. e, A two-bead assay on a set of 39 COVID-19-positive samples and 55 prepandemic samples. The dot colour indicates the sample volume: 100 nl is red, 10 nl is green and 1 nl is blue. Each dot is the mean of 12 replicates (3 sample volume replicates and 4 target replicates per sample volume). The targets were: COVID-19 spike S1, spike S1 RBD, spike S2, nucleocapsid and negative (no target). f, A heat map of the values of the 12 replicates for each red dot shown in e.