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 antibody at that titre.
Interpretation: a rising titre (≥4-fold) between paired acute and convalescent sera is the most reliable evidence of current infection; a single high titre above the local baseline is only suggestive.
Limitations: cross-reactions with other Salmonella serotypes and unrelated febrile illnesses; anamnestic reaction (non-specific rise in titre from an unrelated current infection in a previously exposed/vaccinated individual); background titres common in endemic populations; reduced sensitivity with early antibiotic treatment or testing too early in illness.
Clinical use: widely used in resource-limited settings as an inexpensive screening tool despite its limitations, though blood/bone marrow culture remains the more reliable diagnostic standard.