Widal test is used in the serological diagnosis of enteric fever. Which antibody titers are considered diagnostically significant in a patient from a non-endemic area without prior typhoid vaccination?
- A O agglutinin titer ≥1:160 and H agglutinin titer ≥1:160 ✓
- B O agglutinin titer ≥1:80 and H agglutinin titer ≥1:80
- C Vi agglutinin titer ≥1:40 — single titre diagnostic
- D H agglutinin titer alone ≥1:320 is sufficient for diagnosis
Explanation
In non-endemic areas and unvaccinated individuals, Widal O agglutinin ≥1:160 and H agglutinin ≥1:160 are conventionally considered significant. A fourfold rise in titer in paired sera (acute and convalescent 2 weeks apart) is more meaningful than a single titre. O antibodies (somatic LPS antigen) appear earlier (1st week), indicate active infection, and fall faster; H antibodies (flagellar) appear later, persist longer, and may reflect past infection or vaccination. In endemic India, baseline titers may be higher, making interpretation difficult — blood culture remains the gold standard.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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