A 38-year-old man from rural Bihar presents with a 3-week history of bloody diarrhoea, tenesmus, and lower abdominal pain. Stool microscopy shows motile trophozoites with ingested RBCs. What is the investigation of choice to confirm the diagnosis of invasive amoebiasis and assess for extraintestinal complications?

## Diagnostic Approach to Invasive Amoebiasis **Key Point:** Serology is the gold standard for confirming invasive *Entamoeba histolytica* infection, particularly when trophozoites with ingested RBCs are already seen on microscopy in a symptomatic patient. ### Why Serology is the Investigation of Choice In this clinical scenario: - The patient has **dysentery** (bloody diarrhoea with tenesmus) — a hallmark of invasive disease - Trophozoites with ingested RBCs on microscopy strongly suggest tissue invasion - Serology becomes **positive in >90% of cases** with invasive amoebiasis (intestinal or extraintestinal) - Serology helps differentiate **pathogenic *E. histolytica*** from non-pathogenic *E. moshkovskii* and *E. dispar* ### Serological Tests Available | Test | Sensitivity (Invasive) | Specificity | Timing | |------|------------------------|-------------|--------| | Indirect haemagglutination (IHA) | 90–95% | 95% | Positive by day 7–10 | | ELISA (IgG) | 90–100% | 95–98% | Positive by day 7–10 | | Immunofluorescence (IFA) | 85–95% | 95% | Positive by day 5–7 | | Gel diffusion precipitin | 70–80% | 95% | Slower, less used | **High-Yield:** Serology is **positive in 90–95% of invasive cases** but only 10% of asymptomatic carriers or non-invasive infections. ### Additional Investigations for Extraintestinal Complications - **Ultrasound abdomen** — to detect liver abscess (most common extraintestinal manifestation) - **CT abdomen** — for staging and localizing abscess - **Serology + imaging** — together confirm invasive disease and guide drainage if needed **Clinical Pearl:** A patient with dysentery + positive serology + imaging evidence of liver abscess has invasive amoebiasis until proven otherwise. Serology is often MORE reliable than stool microscopy in invasive disease because trophozoites may not be shed in stool if abscess is walled off. **Mnemonic: SEROLOGY for SYSTEMIC** — When *E. histolytica* invades tissue (dysentery, abscess), the immune system responds with antibodies; serology detects this humoral response.