A patient with typhoid fever has a Widal test done on day 10 of illness showing O antigen titre of 1:160 and H antigen titre of 1:320. No paired serum is available. What is the correct interpretation?

• A O titre 1:160 is diagnostic of active Salmonella Typhi infection in an endemic area; H titre 1:320 suggests past infection or vaccination
• B H titre is more diagnostically significant than O titre; a single titre of 1:320 H confirms typhoid
• C The Widal test is unreliable in endemic areas; a single O titre of 1:160 is suggestive but not diagnostic; fourfold rise in paired sera is the gold standard; blood culture remains the gold standard overall ✓
• D O titre 1:80 is the cut-off for diagnosis in non-endemic areas; in endemic areas the O titre is irrelevant

Explanation

The Widal test has significant limitations in endemic areas (e.g., India): background titres from prior subclinical infection, vaccination, and cross-reactions with other Salmonella species make single-sample results unreliable. The gold standard for interpretation is a fourfold rise in O antigen titre between acute (day 7–10) and convalescent (day 21–28) sera. In endemic areas, single O titres ≥1:160 are considered suggestive (not diagnostic). H antigens persist longer and may be elevated due to prior TAB vaccination or past infection, making them less useful for acute diagnosis. Blood culture in the first week of illness remains the gold standard for typhoid diagnosis.

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

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