Enterovirus IgM detection: specificity of mu-antibody-capture radioimmunoassays using virions and procapsids of Coxsackie B virus.

A predominantly type-specific mu-capture radioimmunoassay (RIA) of IgM antibodies to Coxsackie B1-B5 (CB1-CB5) viruses was previously described (Frisk et al., 1984). The present study is concerned with the specificity of this assay, using as antigen different strains of one serotype (CB5) and procapsids of two serotypes (CB3 and CB5). Eight strains of CB5 virions were tested against acute and/or convalescent sera from 10 patients from whom CB5 had been isolated. Seven patients' sera were tested against their own strain. The frequency of IgM-positive patients varied from 9 of 10 (90%) to 5 of 10 (50%). In three cases the highest titres were obtained with the patients' own strain. When sera from patients with other enterovirus infections were tested against the CB5 strains, heterotypic titres were obtained to a certain extent (0-15.6%). The strains giving a high frequency of homotypic titres varied concerning heterotypic reactions. It is concluded that the choice of strain is important if a high frequency of homotypic titres with no or only a few heterotypic reactions is to be obtained. When procapsids were used as antigen, both homotypic and heterotypic titres were seen to a large extent. All patients with homotypic IgM against CB3 or CB5 virions showed IgM against the CB3 or the CB5 procapsids, respectively. When sera from patients with other enterovirus infections were tested, IgM was found in 54 of 93 patients (58%) with use of the CB3 procapsid and in 52 of 87 patients (60%) with the CB5 procapsid. It was often not the same patients who showed IgM against the two different procapsids. When both procapsids were used, IgM positivity was found in 62 of 81 patients (77%) with other enterovirus infections. It is concluded that the use of two or more procapsids in combination is of value for the diagnosis of a recent or current enterovirus infection.