The SARS‐CoV‐2 IgG, IgA, and IgM ELISA kits manufactured by Virotech Diagnostics for Gold Standard Diagnostics are qualitative assays that detect separately IgG, IgA, and IgM antibodies to the Nucleocapsid protein (N) of the SARS‐CoV‐2 virus. The assays were conducted according to the manufacturer's instructions. The assays use a 1:100 serum dilution, controls, and calibrator, and the reactions are read at a wavelength of 450 nm with a reference wavelength of 620nm. Tests are reported in units and are considered positive if the sample optical density/cut off 10× is greater than 11.0 units; equivocal if 9.0–11.0 units; and negative less than 9.0 units. For analysis of our data, we included samples in the equivocal category as positive. Although the assays are qualitative, we use the units obtained to compare intensities of reactivity. Specificity provided in the manufacturer's package insert was 100% for IgG and IgA, and 100% and 98.7% for IgM when testing healthy U.S. and German blood donors, respectively. Cross‐reactivity was observed by the manufacturer in 1 and 8 of 110 samples from patients with viral and bacterial respiratory pathogens for IgG and IgM, respectively, whereas no cross‐reactivity was observed in IgA ELISA. These assays have been submitted by the manufacturer to the FDA for EUA.

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