Widal

Widal refers to the Widal test, a serological method historically used to diagnose typhoid fever by detecting antibodies against Salmonella enterica serovar Typhi. The test is named after Georges-Ferdinand Widal, who introduced it in the late 19th century. It is typically performed as a slide or tube agglutination assay using patient serum and killed bacterial antigens, commonly representing the O (somatic) and H (flagellar) antigens of S. Typhi, and, in some formulations, the Vi capsular antigen.

In practice, a patient’s diluted serum is mixed with standardized Salmonella antigens. If specific antibodies are present, visible agglutination occurs, and the highest dilution showing agglutination is recorded as the antibody titer. Results are interpreted in the context of regional baseline titers, patient age, and illness duration. A fourfold rise in titer between acute and convalescent sera supports recent infection; a single titer is less definitive.

Limitations of the Widal test are significant. Cross-reactivity with antibodies to other Salmonella serovars or prior exposure, vaccination, or previous typhoid infection can yield false positives. Conversely, early in illness, antibodies may not yet be detectable, producing false negatives. Baseline titers vary by geography and population, complicating interpretation. Because of these limitations, the Widal test is often considered a supplemental tool rather than a definitive diagnostic test.

Today, blood culture remains the gold standard for typhoid diagnosis in many settings, while newer serological and molecular tests have increasingly supplanted the Widal test, especially in non-endemic areas.