Although the incidence of typhoid fever in this country has been decreasing,1 sporadic outbreaks continue to occur.2,3 Increasing numbers of military personnel and private citizens are being exposed to typhoid during foreign travel to areas where the disease is endemic; consequently, the diagnosis of typhoid fever remains an important problem. The definitive diagnostic procedure is isolation of Salmonella typhosa from blood, stool, or occasionally, urine, but serologic tests are still widely used. This communication will briefly discuss the following aspects of serologic tests for typhoid fever: the antigens used, cross-reactivity with other salmonellae, traditional interpretive criteria of test results, serologic response in the vaccinated patient, the effect of antibiotic treatment on antibody response, and the Vi test for identifying typhoid carriers.
Diagnostic tests for typhoid bacillus, such as Widal's reaction, are tube dilution tests which measure agglutinating antibodies against O and H antigens. The Vi antigen is used