Figure 2.

True negative (pre-COVID, acquired prior to December 2019) patient saliva samples were sourced from a commercial vendor and assessed using the BRAVO assay. Although variation in detectable salivary IgA existed across the samples (potentially due to polyreactive IgA or cross-reactivity with prior coronavirus exposure), the results clustered around a mean of 43.2 AU. We set a threshold cut-off to separate negative from positive samples where 95% of samples were negative. This value was set at 75 AU, or 1.65 standard deviations above the negative value mean (indicated by the broken red line).