What is the purpose of a peritoneal fluid biopsy in patients with ascites?

Purpose of Peritoneal Fluid Biopsy in Patients with Ascites

Peritoneal fluid biopsy is the gold standard diagnostic procedure when the cause of ascites remains uncertain, allowing for histological examination and culture to definitively diagnose conditions like tuberculous peritonitis and peritoneal carcinomatosis.1

Primary Indications for Peritoneal Fluid Biopsy

• Undiagnosed ascites: When routine ascitic fluid analysis (including SAAG, cell count, culture) fails to establish a diagnosis, peritoneal biopsy becomes necessary to determine etiology 1

• Suspected tuberculous peritonitis: Laparoscopy with peritoneal biopsy and mycobacterial culture of tubercles is the most rapid and accurate diagnostic method for tuberculous peritonitis 1

• Suspected malignancy: When peritoneal carcinomatosis is suspected but ascitic fluid cytology is negative or inconclusive 1

• Mixed ascites: Approximately 5% of patients have two or more causes of ascites formation (e.g., cirrhosis plus peritoneal carcinomatosis or tuberculous peritonitis) requiring biopsy for definitive diagnosis 1

Diagnostic Algorithm for Ascites Evaluation

1. Initial evaluation: Perform diagnostic paracentesis with ascitic fluid analysis including SAAG, total protein, cell count and differential 1

2. Second-line tests: Based on clinical suspicion, perform targeted tests:

   • For suspected infection: Cell count, bacterial culture in blood culture bottles 1
   • For suspected malignancy: Cytology (sensitivity ~82.8% for first sample) 1
   • For suspected TB: ADA assay, AFB smear and culture (sensitivity of fluid culture ~50%) 1

3. When to perform peritoneal biopsy:

   • When ascitic fluid analysis is inconclusive 1
   • When TB is suspected but ADA is negative (especially in cirrhotic patients where ADA sensitivity may be lower) 1
   • When malignancy is suspected but cytology is negative 1

Specific Diagnostic Yield by Condition

• Tuberculous peritonitis: Peritoneal biopsy with histopathology showing caseating granulomatous inflammation has significantly higher diagnostic yield (22.2%) compared to ascitic fluid AFB smear (0-86%) and culture (20-83%) 2, 1

• Malignancy: Peritoneal biopsy can detect metastatic adenocarcinoma (13.3% of cases) and rare conditions like peritoneal mesothelioma that may be missed by fluid cytology alone 2

• Non-specific inflammation: In many cases (40%), peritoneal biopsy may show non-specific chronic inflammation, which helps exclude specific pathologies 2

Safety Considerations

• Peritoneal biopsy is generally safe with minimal complications 2

• Common post-procedure issues include pain (91.1%) and mild swelling (53.3%) at the biopsy site 2

• Serious complications like intraperitoneal bleeding are rare (reported in only 1 out of 45 patients in one study) 2

Limitations and Caveats

• Approximately 22.2% of peritoneal biopsies may be non-representative or inconclusive 2

• When performing peritoneal biopsy, samples should be obtained for both histopathology and culture to maximize diagnostic yield 1

• Blind needle peritoneal biopsy has been largely replaced by laparoscopic-guided biopsy which allows direct visualization and targeted sampling of abnormal peritoneum 1

• CA125 levels are elevated in all patients with ascites regardless of etiology and decrease when ascites is controlled; therefore, CA125 is not helpful in differential diagnosis and should not be used 1

Alternative Diagnostic Approaches

• For suspected tuberculous peritonitis, PCR testing for mycobacteria may provide more rapid diagnosis than traditional culture methods 1

• For suspected secondary peritonitis from perforated viscus, ascitic glucose (<50 mg/dL), LDH (higher than serum), CEA (>5 ng/mL), or alkaline phosphatase (>240 U/L) can be helpful diagnostic markers 1