Second, despite the fact that Roche ECLIA measures total antibodies (IgG and IgM) binding to the RBD domain of S1 subunit protein instead of the native full-length S protein, we found excellent PPA between the results returned by this assay and by FCI (IgG, IgA or IgG/IgA), ranging between 96.1 and 97.7%, and strong inter-rater agreement (k value > 0.8), reinforcing the idea that humoral immune response against SARS-CoV-2 following natural infection is mainly directed towards RBD3–5. The gene discussed is PROS1; the disease is infection.