The Widal test in enteric fever measures agglutinating antibodies against Salmonella typhi O and H antigens. A rising titre of O antibody is considered more diagnostically significant than rising H antibody because:

• A O antigen is thermolabile and destroyed during febrile illness, preventing false positives from prior infection
• B H antibodies are non-specific and cross-react broadly with other Salmonella serotypes and prior TAB vaccination
• C O antibodies are IgM class, indicating acute active infection, while H antibodies are IgG, reflecting past infection or vaccination ✓
• D O antibodies appear earlier (day 6–8) and are more genus-specific; H antibodies persist longer and are less indicative of current disease

Explanation

O antibodies in the Widal test are primarily IgM (somatic antigen response), which are class-specific to acute infection and typically appear by the end of the first week of illness; a fourfold rise in titre (or a single titre ≥1:160 O in endemic areas) is diagnostically significant. H antibodies are predominantly IgG (flagellar antigen response), which persist for years after infection or TAB vaccination, making them less useful for diagnosing current infection. Rising or high H titres alone can reflect prior exposure. The combination of O ≥1:160 with rising titre is the meaningful Widal result in acute enteric fever.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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Written and medically reviewed by the StethoPrep medical team.