How to diagnose tuberculous (TB) peritonitis clinically and through laboratory tests in a patient with symptoms such as abdominal pain, ascites, and weight loss, and possibly a history of tuberculosis (TB) or immunocompromised status?

Diagnosis of Tuberculous Peritonitis

Diagnose tuberculous peritonitis by combining clinical suspicion (abdominal pain, ascites, weight loss) with ascitic fluid adenosine deaminase (ADA) levels ≥32-40 U/L in non-cirrhotic patients or ≥27 U/L in cirrhotic patients, which has 100% sensitivity and 96.6-100% specificity, and confirm with laparoscopy and peritoneal biopsy showing caseating granulomas when non-invasive tests are inconclusive. 1, 2

Clinical Presentation

The diagnosis begins with recognizing the characteristic but nonspecific clinical pattern:

Cardinal Symptoms (in order of frequency)

• Abdominal pain (65-100% of cases) 3, 4, 5
• Abdominal distension/ascites (91-96% of cases) 4, 5, 6
• Weight loss (36-82.6% of cases) 3, 4
• Fever (60-75% of cases) 3, 4, 5
• Loss of appetite (73-87% of cases) 4, 5

Key Clinical Features

• Symptoms typically present for 1-12 months before diagnosis (mean 3.1-3.6 months), distinguishing this from acute bacterial peritonitis 4, 7
• Only 15-19% have abnormal chest X-ray suggesting prior tuberculosis 5, 7
• Up to 85% have no pulmonary involvement, so absence of lung findings does not exclude the diagnosis 3
• Tuberculin skin test (purified protein derivative) is usually negative in immunocompromised patients 3

Laboratory Diagnosis

Ascitic Fluid Analysis (First-Line)

Ascitic fluid ADA is the single most useful non-invasive test:

• ADA ≥32-40 U/L in non-cirrhotic patients: 100% sensitivity, 96.6-100% specificity 1, 2, 8
• ADA ≥27-32 U/L in cirrhotic patients: 91.7-100% sensitivity, 92-93.3% specificity 1, 2, 8
• ADA <40 U/L effectively excludes tuberculosis 8

Ascitic Fluid Characteristics

• Exudative ascites with serum-ascites albumin gradient (SAAG) <1.1 g/dL 4
• Lymphocyte predominance in ascitic fluid (sensitivity 0-86%) 1, 7
• Ascitic fluid protein typically elevated 4

Microbiological Tests (Low Yield)

• Acid-fast bacilli (AFB) smear: only 0-86% sensitivity 1, 2
• Mycobacterial culture: only 20-83% sensitivity 1, 2
• PCR for M. tuberculosis DNA: variable results (positive in 9-52% of cases) 4, 9, 7

Critical Pitfall: Do not delay treatment while awaiting culture results given the poor sensitivity of microbiological tests 2

Imaging Findings

CT/Ultrasound Features

• Free or loculated ascites present in 30-100% of cases 3
• Variable tomographic density (20-45 Hounsfield units) depending on disease stage 3
• Multiple mesenteric lymph nodes with peripheral enhancement and central hypodensity 3
• Lipohydric level in association with necrotic lymph nodes is highly specific for tuberculous ascites 3
• Omental involvement in 65.2% of cases 4
• High-density ascites on imaging contributes to diagnosis 7

Important Limitation: CT scan is not sufficiently sensitive or specific alone for diagnosis 3

Definitive Diagnosis: Laparoscopy with Biopsy

When ADA is unavailable or results are equivocal, laparoscopy with directed peritoneal biopsy is the gold standard:

Laparoscopic Findings

• Scattered yellowish-white nodules (1-5 mm) on peritoneal surfaces with filmy adhesions are pathognomonic 6
• Laparoscopy suggests tuberculous peritonitis in 95.5% of cases 6
• Three main presentations: ascitic (most common), plastic/dry (3-4.5%), and glandular 3, 5

Histopathology

• Caseating granulomas: 76% of biopsies 5
• Non-caseating granulomas: 20% of biopsies 5
• Histologic confirmation achieved in 97% of laparoscopic cases 6

Major Complication: Non-fatal colon perforation can occur but is rare 6

Diagnostic Algorithm

Step 1: Clinical Suspicion

• Patient with chronic abdominal pain, ascites, weight loss, and fever for >1 month 4, 5
• Consider epidemiology: more common in low-resource settings, immunocompromised patients, and women 3, 4

Step 2: Ascitic Fluid Analysis

• Perform diagnostic paracentesis immediately 3
• Measure ADA levels (if available) 1, 2
• Check cell count with differential (lymphocyte predominance) 1, 7
• Calculate SAAG (should be <1.1 g/dL) 4
• Send for AFB smear and culture (despite low yield) 1, 2

Step 3: Imaging

• CT or ultrasound to identify ascites, lymphadenopathy, and omental involvement 3, 4

Step 4: Definitive Diagnosis

• If ADA ≥32-40 U/L (or ≥27 U/L in cirrhotics) AND clinical picture consistent: diagnose tuberculous peritonitis 1, 2
• If ADA unavailable or equivocal: proceed to laparoscopy with peritoneal biopsy 5, 6

Step 5: Exclude Secondary Peritonitis

Differentiate from secondary bacterial peritonitis using:

• Total protein >1 g/dL, LDH > upper limit of normal, glucose <50 mg/dL (suggests secondary peritonitis) 1
• Multiple organisms on Gram stain/culture (suggests secondary peritonitis) 1
• PMN to total leukocyte ratio ≤75% (suggests malignancy rather than bacterial peritonitis) 1

Special Considerations

In Endemic Areas

• Empiric antituberculosis treatment is recommended for exudative ascites with elevated ADA after excluding malignancy, uremia, trauma, and bacterial peritonitis 2

In Non-Endemic Areas

• Do not start empiric treatment without stronger diagnostic evidence (lower positive predictive value when prevalence is low) 2

In Resource-Limited Settings

• Clinical diagnosis supported by basic laboratory tests and ultrasound when CT unavailable 3
• Tuberculin skin test, radiologic findings, and history of TB exposure contribute to diagnosis when invasive methods unavailable 7