Antibody response to somatic antigen of Salmonella typhi in areas endemic and non-endemic for typhoid fever.

In sera obtained between the 6th and the 30th day from 16 Austrian and 26 Hong Kong patients with culturally verified typhoid fever, agglutinating antibodies (microagglutination test) at significant titers were detected in 93% of the Austrian (median titer: 640) but in only 50% of the Hong Kong patients (median titer: 240). Similar results (93% and 54% positive sera respectively) were obtained for specific IgM as assessed by the enzyme-linked immunosorbent assay (ELISA) using lipopolysaccharide of S. typhi as antigen (median relative titer: 0.32 and 0.21 respectively). In contrast, specific IgG at significant concentrations were found in only 69% of the Austrian (median relative titer: 0.16) but 88% of the Hong Kong sera (median relative titer: 0.71). The (IgM-detecting) microagglutination test may be sufficiently diagnostic for typhoid fever in nonendemic areas such as Austria. In endemic regions like Hong Kong, however, tests indicative for early specific IgG are indispensable for serological diagnosis of the disease. The ELISA proved useful and is an example for such tests.