Figure 3

Venous whole blood stored on filter paper shows comparable serum neutralisation of SARS-CoV-2 spike to paired sera aliquots. Pseudo-virus particles (PVP) expressing SARS-CoV-2 spike were used to measure the neutralisation capacity of paired human sera with venous blood eluates that were stored on filter paper as dried blood spots (VDBS). Neutralisation activity was defined as the serum dilution that reduced PVP infectivity by 70% (IC70). As whole blood contains approximately 55% serum this was accounted for when calculating IC70 values for VDBS eluates. (a) Violin plot displays on the y-axis PVP neutralisation as IC70 values. The x-axis shows paired sample types (n = 10) with sera represented by orange circles and 3 pipetted volumes of VDBS (15 μL, 25 μL & 50 μL) represented by pink circles. The dotted lines across the plot represent the lower limits of detection, with the limit for sera in orange and VDBS eluates in pink. A repeated measures one-way ANOVA test was run to compare the mean IC70 value for sera against the mean IC70 values for VDBS eluates. A significant difference was found (n = 10, p < 0.0001) and a Holm-Šidák’s multiple comparisons test identified it to be between the mean sera and mean 15 μL VDBS eluates IC70 values (p < 0.0001). No significant differences (ns) were found between the mean IC70 values for sera and the 25 μL and 50 μL VDBS eluates (p > 0.05) (b-d) Scatter plots show the IC70 values for sera plotted against the IC70 values for VDBS eluates (b) 15 μL VDBS (c) 25 μL and (d) 50 μL VDBS. Simple linear regression analysis found significant relationships between IC70 values for sera and 15 μL VDBS eluates [n = 10, R² = 0.6780, p = 0.0034], 25 μL VDBS eluates [n = 12, R² = 0.7429, p = 0.0003] and 50 μL VDBS eluates [n = 12, R² = 0.8655, p < 0.0001]. The dotted lines represent the upper and lower 95% confidence intervals (CI) of the line of best fit. (3e–3g.) Bland–Altman plots display on the x-axes the average IC70 values for sera and VDBS eluates (e) 15 μL (f) 25 μL and (g) 50 μL and the difference between the IC70 values on the y-axes. The red lines represent bias, and the dotted lines represent the upper and lower 95% CI.