In Widal's test for typhoid fever, which antibody titre against which antigen is considered diagnostically significant in an unvaccinated individual from a non-endemic area?
- A Anti-'H' (flagellar) antibody ≥1:80
- B Anti-Vi capsular antibody ≥1:40
- C Anti-'O' (somatic) antibody ≥1:160 or a fourfold rise in paired sera ✓
- D Anti-'H' antibody ≥1:160 in a single sample
Explanation
In the Widal test, anti-O (somatic) antibody titre ≥1:160 in a single sample, or a fourfold rise between acute and convalescent paired sera, is considered diagnostically significant for typhoid fever in an unvaccinated individual from a non-endemic area. Anti-O antibodies appear earlier (end of first week), are shorter-lived, and are more specific for active infection. Anti-H (flagellar) antibodies are less specific as they persist longer post-infection and post-vaccination, and are commonly elevated in endemic populations. Anti-Vi titres are used for detecting chronic carriers.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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