Whenever an assay is utilized in a low prevalence population the positive predictive value for the assay will be very low. We suggest testing samples on the S1 and S2 spike plates first. If a sample is positive, the same sample could be tested on the nucleoprotein plate, RBD + M plate and the seasonal coronavirus plates. If the sample is positive on the spike plate and negative on the others, it would indicate that the sample has antibodies to the HEK vector or the Fc tag as the others are different vectors and tags. If the sample is positive on all, it could indicate a cross reaction to other coronavirus, otherwise it is a non-specific sera.
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