The Solsten SARS-CoV-2 Neutralizing Antibody ELISA is a 1-step competitive assay, where neutralizing antibodies in the blood sample competes with the peroxidase-labeled human ACE-2 receptor to bind to the immobilized receptor-binding domain of the virus’ Spike protein (S1-RBD). The reduction in color signal is proportional to the antibody-derived protection (titer) against the virus.