The Widal test is a serological agglutination assay used to detect antibodies against the O and H antigens of Salmonella enterica serovar Typhi and, to a lesser extent, S. Paratyphi. It has historically been used to support the diagnosis of typhoid fever by measuring agglutinins in a patient’s serum.
Principle and interpretation
In the Widal test, serum from a suspected patient is serially diluted and mixed with standardized suspensions of killed Salmonella O (somatic) and H (flagellar) antigens. If antibodies are present, visible agglutination occurs. O antibodies (TO agglutinins) appear earlier in infection and typically rise during the first week, whereas H antibodies (TH agglutinins) rise later and persist longer. The test can be performed by slide agglutination for screening or by tube agglutination to determine endpoint titres. Results are expressed as the highest dilution showing agglutination. Interpretation requires knowledge of baseline antibody titres in the local population because low-level agglutinins may be present due to prior infection or vaccination. A fourfold rise in titre between acute and convalescent sera is more indicative of active typhoid fever than a single measurement.
Clinical use and limitations
Although simple and inexpensive, the Widal test has significant limitations. Cross-reactivity with non‑typhoidal Salmonella, malaria, dengue and other infections can produce false-positive results. Prior vaccination or exposure may also elevate baseline titres. In endemic areas, interpretation is further complicated by background antibody prevalence; thus a single cut-off titre may not be reliable. Blood culture remains the gold standard for diagnosing typhoid fever, and bone marrow culture has higher sensitivity. Despite these issues, the Widal test is still used in resource-limited settings where culture facilities are unavailable, often alongside clinical assessment and other laboratory findings. Newer diagnostic methods such as rapid immunoassays and nucleic acid tests are replacing Widal in many laboratories.
The Widal test provides a historical example of serological diagnosis of bacterial infection but requires cautious interpretation and is increasingly supplemented or replaced by more specific methods.
Related Terms: Typhoid fever, Salmonella Typhi, Agglutination, Serology, Blood culture