Widal test in a patient at day 10 of enteric fever shows O agglutinins 1:320 and H agglutinins 1:160. A single-sample interpretation in an endemic area should be:
- A Significant rise in O titres is more indicative of active infection than H titres, but single-sample interpretation is unreliable in endemic areas ✓
- B Diagnostic because O titre ≥1:80 confirms active infection
- C H agglutinins alone diagnose typhoid fever
- D Widal test is 100% specific and a titre of 1:320 always indicates active typhoid
Correct answer: A. Significant rise in O titres is more indicative of active infection than H titres, but single-sample interpretation is unreliable in endemic areas
Explanation
In typhoid endemic areas, baseline O and H titres are elevated from prior exposure or vaccination; a fourfold rise in paired samples (acute and convalescent) is the gold standard for interpretation. O agglutinins (antibodies to somatic antigen) rise earlier and fall faster, making them more indicative of active infection; H agglutinins (flagellar) persist longer and rise after vaccination. Single-sample titres are unreliable and not diagnostic in endemic regions; 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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