What is the most likely diagnosis and appropriate work‑up for a patient with abdominal pain, weight loss, fever, new‑onset ascites and risk factors such as tuberculosis exposure, HIV infection, chronic steroid use, or residence in a TB‑endemic area?

Diagnosing Disseminated Tuberculosis in the Gastrointestinal Tract with Ascites

In patients with abdominal pain, ascites, weight loss, fever, and TB risk factors (endemic residence, HIV, immunosuppression), perform diagnostic paracentesis immediately and measure ascitic fluid adenosine deaminase (ADA), lymphocyte count, acid-fast bacilli (AFB) smear/culture, and cell count/differential, while simultaneously obtaining abdominal imaging and pursuing tissue diagnosis via laparoscopy with peritoneal biopsy when initial fluid studies are non-diagnostic. 1, 2

Initial Clinical Recognition

High-Risk Features Mandating TB Work-Up

• Residence in or immigration from TB-endemic areas (South Asia, sub-Saharan Africa, Latin America) is the strongest epidemiologic clue 1, 3
• HIV infection, chronic steroid use, or other immunosuppression dramatically increases risk 1
• Constitutional symptoms: fever (64%), night sweats (48%), and weight loss (47-72%) occur in the majority 4, 5
• Abdominal pain (84-93%) and ascites (35-42%) are the most common presenting features 4, 5

Physical Examination Priorities

• Test for shifting dullness (83% sensitivity) to confirm ascites; approximately 1,500 mL must be present before flank dullness is detectable 1, 2, 6
• Palpate for abdominal tenderness and masses (present in 42% of TB peritonitis cases) 5
• Assess for splenomegaly and mesenteric lymphadenopathy on deep palpation 2

Mandatory Paracentesis Protocol

Immediate Ascitic Fluid Analysis

Perform diagnostic paracentesis in all patients with new-onset ascites and TB risk factors; coagulopathy is NOT a contraindication (major bleeding occurs in only ~1% of procedures). 1, 2, 7

Core Ascitic Fluid Tests

• Cell count with differential: Lymphocyte predominance (>50% lymphocytes) strongly suggests TB peritonitis, but neutrophil count >250 cells/mm³ indicates concurrent spontaneous bacterial peritonitis requiring immediate antibiotics 1, 2
• Adenosine deaminase (ADA):
  • ADA >32-40 U/L has 91.7-100% sensitivity and 92-96.6% specificity for TB peritonitis in non-cirrhotic patients 1
  • In cirrhotic ascites, use lower cutoff of 27 U/L (100% sensitivity, 93.3% specificity) 1
  • ADA is the single most useful biochemical test for TB peritonitis 1
• AFB smear and mycobacterial culture: Inoculate at least 10 mL into blood culture bottles at bedside before any antibiotics; culture sensitivity is 50-80% but smear sensitivity is only 0-7% 1, 8
• Serum-ascites albumin gradient (SAAG): Calculate to distinguish portal hypertension (SAAG ≥1.1 g/dL) from non-portal causes; TB peritonitis typically has SAAG <1.1 g/dL unless concurrent cirrhosis exists 1, 2
• Total protein: TB ascites is exudative (protein >2.5 g/dL) in 95% of cases 8

Advanced Molecular Testing

• TB PCR on ascitic fluid or peritoneal biopsies provides rapid diagnosis when available and improves discrimination from Crohn's disease 1

Essential Imaging Studies

Abdominal Ultrasound (First-Line)

• Mesenteric thickness ≥15 mm with increased echogenicity combined with mesenteric lymphadenopathy is characteristic of early abdominal TB (present in 100% of one series) 9
• Assess for ascites, dilated small bowel loops, omental thickening, and hepatosplenomegaly 5, 9
• Ultrasound detected abnormalities in 66% of abdominal TB cases 5

CT Abdomen with Contrast

• CT reveals abnormalities in 100% of abdominal TB cases when performed, making it superior to ultrasound 5
• Look for: mesenteric/retroperitoneal lymphadenopathy, peritoneal thickening, omental caking, bowel wall thickening, and ascites 3, 5

Chest Radiograph

• Obtain in all suspected cases; concomitant pulmonary TB is present in 32-64% of abdominal TB patients 4, 5, 8
• Normal chest X-ray does NOT exclude abdominal TB 5

Tissue Diagnosis When Fluid Studies Are Non-Diagnostic

Laparoscopy with Peritoneal Biopsy (Gold Standard)

Laparoscopy with biopsy and mycobacterial culture of peritoneal tubercles is the most rapid and accurate diagnostic method, achieving 75-85% diagnostic yield. 1, 5, 8

• Perform when: ADA is equivocal, AFB culture is pending, or clinical suspicion remains high despite negative initial studies 1
• Histopathology reveals non-necrotizing or caseating granulomas in the majority 3, 5
• AFB are visualized in only 34% of tissue specimens, but culture positivity reaches 60% 5

Alternative Tissue Sampling

• Ultrasound-guided biopsy of omental masses or mesenteric lymph nodes when laparoscopy is unavailable 3, 9
• Colonoscopy with ileocecal biopsy if intestinal TB is suspected (look for transverse ulcers, patulous ileocecal valve, and <4 colonic segments involved) 1

Critical Diagnostic Pitfalls

Common Errors to Avoid

• Do not rely on AFB smear alone—sensitivity is near 0% in ascitic fluid 1, 8
• Do not wait for culture results (take 6-8 weeks) before initiating treatment if clinical suspicion and ADA are high 1, 8
• Do not order serum CA-125—it is elevated in all causes of ascites and leads to unnecessary gynecologic referrals 1
• Do not assume normal chest X-ray excludes TB—50% have no pulmonary involvement 5, 8

Distinguishing TB from Crohn's Disease

When ileocecal involvement is present, TB is suggested by: 1

• Transverse ulcers, patulous ileocecal valve, <4 colonic segments involved
• Positive QuantiFERON-Gold test and typical pulmonary lesions
• Absence of anorectal lesions, longitudinal ulcers, and ASCA antibodies (which favor Crohn's)

Diagnostic Algorithm Summary

1. High clinical suspicion (endemic exposure + ascites + constitutional symptoms) → immediate paracentesis 1, 2
2. Ascitic fluid analysis: ADA, lymphocyte count, AFB culture (bedside inoculation), SAAG, total protein 1, 2
3. Simultaneous imaging: Abdominal ultrasound or CT (CT preferred if available) + chest X-ray 5, 9
4. If ADA >32 U/L (or >27 U/L in cirrhosis) + lymphocytic ascites → presume TB peritonitis and initiate treatment 1
5. If ADA equivocal or negative but suspicion remains high → proceed to laparoscopy with peritoneal biopsy 1, 8
6. Therapeutic trial of anti-TB drugs is justified only when tissue diagnosis is impossible and clinical/biochemical features are highly suggestive 4, 5

Confirmatory Evidence of Response

• Repeat ultrasound at 1,3,6, and 12 months shows regression of mesenteric thickening and lymphadenopathy in successfully treated cases 9
• Clinical improvement (resolution of fever, weight gain, decreased ascites) typically occurs within 2-4 weeks of appropriate therapy 5, 8