Significance of serological testing in typhoid fever

• O antigen: somatic antigen. A rising anti-O titre may suggest recent or active infection.
• H antigen: flagellar antigen. It tends to appear later and can persist longer, so it may reflect previous infection, vaccination, or exposure rather than acute disease.
• Some assays may also test Vi antibody, which can support investigation of chronic carriers but is not diagnostic by itself.

Potential roles

1. Supportive evidence when typhoid is clinically suspected and culture facilities are unavailable or blood culture is negative.
2. Epidemiological or retrospective assessment of exposure in selected settings.
3. A fourfold rise in titre between acute and convalescent serum samples is more meaningful than a single positive result.
4. Rapid antibody tests such as Typhidot and TUBEX can give quicker results, but their diagnostic performance varies by setting.

Major limitations

• Serology does not definitively diagnose acute typhoid fever.
• In endemic areas, many healthy people have baseline antibodies from prior infection, repeated exposure, or vaccination.
• Cross-reactions may occur with other Salmonella infections and illnesses such as malaria or other febrile infections.
• Antibodies may be absent early in illness, causing false-negative results.
• A single Widal titre has no universal diagnostic cutoff. It must be interpreted against local baseline titres and the clinical context.
• Positive results may lead to overdiagnosis and unnecessary antibiotic use.

Clinical conclusion