Next Page

Laboratory Diagnosis

The diagnosis of typhoid fever is confirmed by isolation of Salmonella typhi bacilli from the blood, stool or urine of a patient with clinical manifestations. Blood culture is surest when done in the first 2 weeks of illness. S. typhi may be seen in feces by the 14th day, and in urine by the 21st day, and may remain there for a long period. In the early stages, there may be a mild leukocytosis, but later a moderate leukopenia and relative lymphocytosis develop.

Specific agglutinins appear in the serum after 7-10 days and can be detected by the Widal reaction, in which agglutination of dead strains of S. typhi reacting with the patient's serum shows increasing titers as the disease progresses. The antisomatic O antibody appears first, rises progressively, and later falls and often disappears after a few months. A three- to four-fold rise in antibody titer is considered significant and indicates acute disease. The antiflagellar H antibody appears later but persists longer, for up to 3 years, and is more reliable than the O antibody titer in determining the type of enteric infection. The Widal test has been criticized because it is nonspecific, but it is still useful when positive in high dilution in an individual who has had no previous attack of typhoid and has not been immunized.

Newer methods of serodiagnosis for typhoid, including indirect hemagglutination, indirect fluorescent Vi antibody, and indirect enzyme linked immunosorbent assay (ELISA) for IgM and IgG antibodies to S. typhi polysaccharide are all superior to the Widal test in both sensitivity and specificity. The Vi agglutination test has limited use as a screening test for the typhoid carrier state. Phage typing is of help in tracing the source of infection and in its control. The use of monoclonal antibodies against S. typhi flagellin and DNA probes to detect the organisms in blood offer further promise.

Back to the Table of Contents

Copyright: Palmer and Reeder

Tropical Medicine Mission Index of Diseases About Tropical Medicine Tropical Medicine Home Page Tropical Medicine Staff