A 32-year-old woman from Mumbai presents with a 3-week history of intermittent fever, anorexia, and abdominal distension. She has no diarrhoea. On examination, she is pale, has hepatosplenomegaly, and a palpable abdominal mass in the right iliac fossa. Blood culture is negative. Widal test shows O antigen 1:80 and H antigen 1:640. Abdominal ultrasound reveals mesenteric lymphadenopathy and a hypoechoic lesion in the terminal ileum. What is the most likely diagnosis and what is the most appropriate investigation to confirm it?

Explanation

## Differential Diagnosis: Enteric Fever vs Ileocaecal TB ### Clinical Scenario Analysis **Key Point:** This patient presents with **chronic fever (3 weeks), hepatosplenomegaly, abdominal mass, and imaging findings of terminal ileal involvement** — a presentation that overlaps enteric fever and ileocaecal tuberculosis. ### Critical Diagnostic Clues | Feature | Enteric Fever | Ileocaecal TB | Crohn's Disease | |---------|---------------|---------------|------------------| | **Duration** | 1–3 weeks (acute) | Weeks to months (insidious) | Months to years | | **Fever pattern** | High-grade, remittent | Low-grade, intermittent | Absent unless complicated | | **Diarrhoea** | Common (>50%) | May be absent | Prominent | | **Widal O antigen** | ≥1:320 (acute) | Negative or low | Negative | | **Widal H antigen** | ≥1:160 (acute) | Negative or low | Negative | | **Blood culture** | Positive (early) | Negative | Negative | | **Terminal ileal mass** | Rare | Common (ileocaecal junction) | Common (skip lesions) | | **Mesenteric lymphadenopathy** | Mild | Prominent | Mild | | **Hepatosplenomegaly** | Common | Uncommon | Rare | | **AFB in biopsy** | Absent | Present (Ziehl-Neelsen) | Absent | ### Why This Is Ileocaecal TB, Not Enteric Fever **High-Yield:** The **low Widal O antigen (1:80) with disproportionately high H antigen (1:640)** is a red flag: - **O antigen 1:80** = below diagnostic threshold for acute enteric fever (typically ≥1:320) - **H antigen 1:640** = elevated but suggests chronic/past infection or **cross-reactivity with TB antigens** - **Negative blood culture after 3 weeks** = argues against active S. typhi bacteraemia - **Palpable abdominal mass + mesenteric lymphadenopathy** = classic for ileocaecal TB **Clinical Pearl:** In endemic TB areas (India), ileocaecal TB is the most common site of abdominal TB. It presents with insidious fever, weight loss, hepatosplenomegaly, and a palpable mass at the ileocaecal junction. Widal test may show low-titre cross-reactivity but is NOT diagnostic. ### Diagnostic Algorithm ```mermaid flowchart TD A["Fever + Abdominal mass + Hepatosplenomegaly"]:::outcome --> B{"Widal O antigen ≥1:320?"}:::decision B -->|"Yes + Blood culture +"| C["Enteric fever"]:::outcome B -->|"No or Low O + High H"|D{"Terminal ileal involvement on imaging?"}:::decision D -->|"Yes"|E["Suspect Ileocaecal TB"]:::outcome E --> F["Colonoscopy with biopsy"]:::action F --> G{"Caseating granuloma + AFB?"}:::decision G -->|"Yes"|H["Ileocaecal TB confirmed"]:::outcome G -->|"No"|I["Consider Crohn's disease"]:::outcome C --> J["Start ceftriaxone"]:::action H --> K["Start anti-TB therapy"]:::action ``` ### Confirmatory Investigation: Colonoscopy with Biopsy **Key Point:** **Colonoscopy with ileoscopy and biopsy** is the gold standard for distinguishing ileocaecal TB from enteric fever and Crohn's disease: 1. **Gross appearance:** - TB: Ulcerated, strictured terminal ileum with "apple-core" appearance - Crohn's: Skip lesions, cobblestone mucosa, fissuring ulcers 2. **Histopathology (Biopsy):** - TB: **Caseating granulomas** (pathognomonic) + AFB on Ziehl-Neelsen stain - Crohn's: Non-caseating granulomas, no AFB - Enteric fever: Mucosal ulceration, Peyer's patch hyperplasia, no granulomas 3. **Culture:** Tissue culture for M. tuberculosis (slow but confirmatory) **Mnemonic: CASEATING** = **C**aseating granulomas + **A**FB stain = **T**uberculosis; **N**on-caseating = **C**rohn's ### Why Ceftriaxone Alone Is Inadequate Starting ceftriaxone without confirming the diagnosis risks: - Delayed TB diagnosis and treatment - Progression to TB peritonitis, obstruction, or perforation - Worsening outcomes if TB is the true diagnosis **Clinical Pearl:** In India, always rule out TB before attributing abdominal pathology to enteric fever, especially if Widal is low-titre or blood culture is negative.