A 38-year-old man from rural Bihar presents with a 3-week history of bloody diarrhea, abdominal pain, and fever. Stool microscopy shows trophozoites with ingested RBCs. What is the most appropriate next investigation to confirm the diagnosis and assess for extraintestinal involvement?

Explanation

## Investigation of Choice for Confirmed/Suspected Invasive Amoebiasis **Key Point:** Serology is the gold standard for diagnosis of invasive E. histolytica (amoebic dysentery and liver abscess), with sensitivity >90% in symptomatic disease. In this patient with bloody diarrhea and RBC-ingesting trophozoites, serology confirms invasive disease and rules out non-pathogenic E. dispar. ### Why Serology in This Clinical Context? **High-Yield:** - Trophozoites with ingested RBCs = invasive E. histolytica (NOT E. dispar) - Serology becomes positive within 7–10 days of symptom onset in dysentery - Sensitivity: ~95% in acute amoebic colitis, ~100% in amoebic liver abscess - Specificity: >90% when combined with clinical + stool findings ### Diagnostic Approach Table | Investigation | Sensitivity in Dysentery | Specificity | Best Use | Limitations | |---|---|---|---|---| | Stool microscopy (trophozoites) | 60–70% | ~100% if RBCs seen | Initial screening | Multiple samples needed; misses E. dispar | | Serology (IHA/ELISA) | 90–95% | >90% | Confirm invasive disease | Negative early (<7 days); positive in past infection | | Repeat stool microscopy | 85–90% (3 samples) | ~100% | Increase yield if initial negative | Time-consuming; does not assess invasion | | Colonoscopy + biopsy | 95% | 100% | Severe disease, rule out IBD | Invasive; not needed if serology + stool positive | | Stool culture | Not applicable | N/A | Not useful for E. histolytica | E. histolytica does not grow on routine culture | **Clinical Pearl:** In endemic areas (Bihar, Assam, Odisha), serology differentiates invasive E. histolytica from asymptomatic E. dispar carriage. The presence of RBC-ingesting trophozoites already suggests invasion, but serology confirms it and guides treatment intensity. **Mnemonic — SEROLOGY in Amoeba:** - **S**ensitive (>90% in dysentery) - **E**xtraintestinal disease marker (100% in liver abscess) - **R**ules out E. dispar - **O**ptimal for invasive confirmation - **L**ate positive (7–10 days) - **O**ver 90% specificity - **G**old standard for systemic disease - **Y**ield improves with clinical severity ### Why NOT the Other Options? - **Repeat stool microscopy:** Increases sensitivity but does NOT confirm invasion or assess for extraintestinal disease (liver abscess). Already have RBC-ingesting trophozoites on first sample. - **Stool culture:** E. histolytica is an obligate parasite; it does not grow on routine bacterial or fungal culture media. - **Colonoscopy with biopsy:** Reserved for severe/fulminant colitis or when diagnosis remains unclear after serology. Invasive and unnecessary if serology + stool microscopy are diagnostic.