A patient's serum is tested with the Widal test on day 10 of fever. The results are: TO 1:160, TH 1:320, BTO 1:40, BTH 1:40. How should these results be interpreted?

• A Reactive BTO and BTH at 1:40 confirm B. paratyphi infection
• B Widal test is positive only for paratyphoid because BTH 1:40 is the elevated titre
• C Significantly elevated TO (somatic, O antigen) and TH (flagellar, H antigen) titres for S. Typhi (≥1:160 is significant) with high TH suggesting ongoing or recent infection; a single elevated titre is not diagnostic — a fourfold rise in paired samples is ideal, but in India titres ≥1:160 (TO) are considered suggestive ✓
• D The test result is non-specific and should be ignored as all four titres are reactive

Explanation

Widal test measures agglutinating antibodies to O (somatic lipopolysaccharide) and H (flagellar) antigens of S. Typhi and S. Paratyphi. TO (O antigen of Typhi) and TH (H antigen of Typhi) titres are elevated here: TO 1:160 and TH 1:320 are significantly elevated (cutoffs vary: ≥1:80–1:160 for O, ≥1:160 for H in endemic India). BTO/BTH 1:40 are within baseline endemic levels and not significant. Rising O titre indicates active infection; rising H titre may persist longer from past infection or vaccination. Limitation: single-sample Widal has poor specificity due to endemic baseline positivity in India. Paired samples showing fourfold rise are far more meaningful clinically.

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

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Written and medically reviewed by the StethoPrep medical team.