Fig. 2: Antibody characterisation and FND Ag-LFT assay analytical sensitivity and specificity.

a Fitted curves of SARS-CoV-2 antibody-antigen binding over a range of antigen concentrations (data points) evaluated using Biolayer-interferometry. Sensor design shown in schematic. b Serial dilution of recombinant nucleocapsid protein with FND Ag-LFT evaluated by lock-in modulation analysis (blue) resulting in a LoD of 17 fM [95%CI: 5.9-41] using an exponential fitting method (fit shown as solid line with 95% CI shaded region). Data points show mean (n = 3 test replicates) and crosses show individual replicates. This was compared to 40 nm gold nanoparticles (green) with an LoD of 16 pM [95%CI: 11–23]. Data points represent mean (n = 3 test replicates) and crosses represent individual replicates. Measurement replicates give a coefficient of variance 73-times smaller than the test replicates coefficient of variance, so measurement variance is negligible, and is excluded from this, and further analysis, but available in the Source Data. LoD shown as solid vertical line with 95% CI (dotted vertical lines) (c) Mean FND signal of recombinant antigens from other human coronaviruses, showing no statistically significant difference from SARS-CoV-2 negatives (one-way ANOVA, p-value = 0.960, F = 0.19, DF = 17, post-hoc Dunnett’s multiple comparisons). Bar represents mean signal and SD (n = 3 test replicates), data points represent individual replicates. (d) Serial dilution of SARS-CoV-2 gamma-irradiated whole virus of both Wild-type (red) and Omicron (orange) strains evaluated by lock-in analysis showing an LoD of 13 TCID₅₀/mL [95% CI: 7.3-22] and 8.8 TCID₅₀/mL [95% CI: 6.6-12], respectively. Data points represent mean (n = 3 test replicates) and crosses represent individual replicates. Exponential fit shown as solid line with 95% CI shaded region. LoD shown as solid vertical line with 95% CI (dotted vertical lines). Source data are provided as Source Data file.