Question
Widal test
Answer
The Widal test is a tube/slide agglutination test used for the serodiagnosis of enteric (typhoid/paratyphoid) fever, detecting agglutinating antibody in patient serum against the O (somatic) and H (flagellar) antigens of Salmonella Typhi (and Paratyphi A/B).
Principle: patient serum is serially diluted and mixed with standardized suspensions of killed Salmonella O and H antigens; visible agglutination at a given dilution indicates the presence of antibody at that titre.
Interpretation:
- A rising titre (≥4-fold increase) between paired acute and convalescent sera (taken 7–10 days apart) is the most reliable evidence of current infection.
- A single high titre above the locally established baseline (which varies by endemic population background titre) is only suggestive, not diagnostic.
- O antibody typically appears earlier and falls faster after recovery; H antibody appears somewhat later but persists longer — the pattern (both O and H elevated versus H alone) can give some indication of the phase of illness.
Limitations:
- Cross-reactions with other Salmonella serotypes and unrelated febrile illnesses (e.g., malaria) can cause false positives.
- Anamnestic reaction — a non-specific rise in titre triggered by an unrelated current infection in a person previously exposed/vaccinated, causing a false-positive result.
- Background titres are common in endemic populations due to prior subclinical exposure/vaccination, requiring locally validated cut-offs.
- Reduced sensitivity in patients who received antibiotics early, or who are tested too early in the illness (before antibody has risen).
Clinical use: despite its limitations, the Widal test remains widely used in resource-limited settings as an inexpensive screening tool, though blood/bone marrow culture remains the more reliable diagnostic standard, and newer rapid tests (Typhidot, IgM dot-EIA) offer improved performance where available.