Value of a single-tube Widal test in diagnosis of typhoid fever in Vietnam

The diagnostic value of an acute-phase single-tube Widal test for suspected typhoid fever was evaluated with 2,000 Vietnamese patients admitted to an infectious disease referral hospital between 1993 and 1998. Test patients h ad suspected typhoid fever and a blood culture positive for Salmonella typh i (n = 1,400) or Salmonella paratyphi A (n = 45). Control patients had a fe brile illness for which another cause was confirmed (malaria [n = 103], den gue [n = 76], or bacteremia due to another microorganism [n = 156] or tetan us (n = 265). An O-agglutinin titer of greater than or equal to 100 was fou nd in 18% of the febrile controls and 7% of the tetanus patients. Correspon ding values for H agglutinins mere 8 and 1%, respectively. The O-agglutinin titer was greater than or equal to 100 in 83% of the blood culture-positiv e typhoid fever cases, and the H-agglutinin titer was greater than or equal to 100 in 67%. The disease prevalence in investigated patients in this hos pital was 30.8% (95% confidence interval, 26.8 to 35.1%); at this prevalenc e, an elevated level of H agglutinins gave better positive predictive value s for typhoid fever than did O agglutinins. With a cutoff titer of greater than or equal to 200 for O agglutinin or greater than or equal to 100 for H agglutinin, the Widal test would diagnose correctly 74% of the blood cultu re-positive cases of typhoid fever. However, 14% of the positive results wo uld be false-positive, and 10% of the negative results would be false-negat ive. The Widal test can be helpful in the laboratory diagnosis of typhoid f ever in Vietnam if interpreted with care.