Persistence of anti-Salmonella O9 IgM as measured by Tubex® TF may contribute to the over-diagnosis of typhoid fever in endemic areas

Abstract

Background: Diagnosing typhoid fever definitively, using bone-marrow, blood or feces culture is either invasive, time-consuming, expensive, or complicated. Hence, clinicians often use clinical presentations and/or rapid diagnostic methods to examine the indirect evidence of Salmonella typhi infections. This study describes the proportion of typhoid fever, challenges of diagnosing typhoid fever, and the kinetics of anti-Salmonella O9 IgM. Methods: During a prospective dengue study in Bandung, Indonesia, 1,431 febrile occurrences were observed among 2,978 adult volunteers over a four-year period. As typhoid fever is endemic and represents a major cause of febrile illness in this study area, acute- and convalescent-phase sera from 964 subjects that had been excluded for dengue and chikungunya infections, were tested with Tubex® TF. To observe the kinetics of anti-Salmonella O9 IgM, we tested sequential sera up to 260 days post-illness from 3 subjects. Results: Based on Tubex® TF, 71 of 964 (7.4%) subjects had anti-Salmonella O9 IgM (score≥4) in their convalescent sera. Anti-Salmonella O9 IgM scores in convalescent sera increased compared to the acute sera in 36 subjects, indicating probable acute typhoid fever. The convalescent scores decreased/remained the same as acute sera in the other 35 subjects, which more likely indicating previous S. typhi infections. Using serially collected sera, it was determined that anti-Salmonella O9 IgM could be detected for a long period after illness. Conclusion: Tubex® TF test should be conducted using paired acute- and convalescent-phase sera to reduce the chance of false positive result that may due to long-lasting anti-Salmonella O9 IgM.