The present invention provides an immunoassay leading to the rapid and simultaneous detection of antibodies to a wide range of infectious pathogens in biological fluids of infected patients. This immunoassay involves the covalent and oriented coupling of fusion proteins comprising an AGT enzyme and a viral antigen on an identifiable solid support (e.g. fluorescent microspheres). The thus obtained antigen-coupled microspheres show enhanced capture of specific antibodies as compared to antigen-coupled microspheres produced by standard amine coupling procedures.