Signs and symptoms so distinctive of typhoid as to render a clinical diagnosis secure are present only in a minority of patients. Particularly at the onset where fever may be the only complaint, typhoid is easily confused with a host of other diseases that share its geographic patterns: malaria, hepatitis, tuberculosis, brucellosis, and typhus, to name a few.

The usual method of diagnosis is the culture of S. typhi from some part of the body. In persons with typhoid fever, culture of bone marrow is positive in 80 to 90 percent, and blood cultures are positive in

70 percent. Cultures of duodenal fluid are positive in 50 percent. Stool and urine cultures have positivity rates of only 30 and 10 percent, respectively.

In 1896, Fernand Widal determined that most persons infected with S. typhi develop antibodies to its cell wall (O antigen) and flagellae (H antigen). Since that time, the Widal test for O and H antibodies in the blood has been used extensively to diagnose typhoid fever. But although the test uses inexpensive materials and is rapid, it is not always reliable. Persons with typhoid fever may never show a rise in antibody levels, and past exposure to S. typhi (such as is common among adults in endemic areas) can mean a positive Widal test, whatever the patient's current ailment. In unimmunized children in endemic areas, however, the Widal test may be of value.

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