A 45-year-old returning traveller from Kerala presents with diarrhoea and frothy, foul-smelling pale stools for 3 weeks. Stool microscopy shows pear-shaped trophozoites with two nuclei and a central axostyle — 12–15 µm in length. Which of the following is the BEST confirmatory test for this diagnosis?
- A String test (Enterotest) with microscopy of duodenal fluid
- B Serum IgG antibodies against Giardia lamblia
- C Colonoscopy with biopsy from sigmoid colon
- D Stool antigen detection (ELISA or immunochromatography) for Giardia-specific antigen (GSA-65 or GDH) ✓
Explanation
The morphology describes Giardia lamblia (G. duodenalis) trophozoites. While String test can sample duodenal mucus directly, it is invasive and reserved for cases negative on stool examination. Stool antigen detection by ELISA or immunochromatographic assay targeting Giardia-specific antigen (GDH — glutamate dehydrogenase, also called GSA-65) has sensitivity >90% and specificity >99%, making it the best confirmatory test currently recommended. Serum IgG cannot distinguish active from past infection. Colonoscopy is inappropriate as Giardia inhabits the small intestine (duodenum/jejunum), not the sigmoid colon.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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