Cytology Criteria for Diagnosing Peritonitis
Cytology is NOT a primary diagnostic tool for bacterial peritonitis—the diagnosis relies on polymorphonuclear leukocyte (PMN) count ≥250 cells/mm³ in ascitic fluid, not cytologic examination. 1
Primary Diagnostic Criterion for Bacterial Peritonitis
The diagnosis of spontaneous bacterial peritonitis (SBP) is established when the ascitic fluid PMN count is ≥250 cells/mm³ (0.25 × 10⁹/L), regardless of culture results. 1 This is the gold standard and does not require cytologic analysis.
Key Diagnostic Parameters:
- PMN count ≥250 cells/mm³: Diagnostic of SBP, even with negative cultures 1
- Culture-negative neutrocytic ascites: Occurs in approximately 50% of SBP cases when older culture methods are used, but up to 80% are positive when fluid is inoculated into blood culture bottles at bedside 1
- Bacterascites: Positive culture with PMN <250 cells/mm³—treat only if symptomatic 1
When Cytology IS Indicated
Cytologic examination should only be ordered when there is high pretest probability of peritoneal carcinomatosis, not for diagnosing infectious peritonitis. 1
Cytology for Malignant Peritonitis:
- Sensitivity: 82.8% on first sample, 93.3% with two samples, 96.7% with three samples 1
- Optimal specimen: 50 mL of fresh warm ascitic fluid hand-carried to laboratory for immediate processing 1
- Clinical context: History of breast, colon, gastric, or pancreatic primary carcinoma 1
- Cytology is positive ONLY in peritoneal carcinomatosis, not in other causes of ascites 1
Distinguishing Secondary from Spontaneous Bacterial Peritonitis
When PMN count is ≥250 cells/mm³, additional ascitic fluid tests help differentiate secondary bacterial peritonitis from SBP:
Secondary Peritonitis Criteria (at least 2 of 3):
- Total protein >1 g/dL 1
- Lactate dehydrogenase (LDH) > upper limit of normal for serum 1
- Glucose <50 mg/dL 1
Additional Secondary Peritonitis Markers:
- Carcinoembryonic antigen (CEA) >5 ng/mL: 92% sensitivity, 88% specificity for gut perforation 1
- Alkaline phosphatase >240 U/L: Suggests perforated hollow viscus 1
- Multiple organisms on Gram stain/culture (frequently including fungi and enterococcus) 1
Specialized Cytologic Findings for Other Conditions
Tuberculous Peritonitis:
- Lymphocytosis in ascitic fluid is typical 1
- Acid-fast bacilli (AFB) smear sensitivity: 0-86% (essentially useless) 1
- Culture sensitivity: 20-83% 1
- Adenosine deaminase (ADA): >32-40 U/L has 100% sensitivity in non-cirrhotic patients; use >27 U/L cutoff in cirrhotic patients 1
Other Specialized Cytologic Findings:
- Bile peritonitis: Bile pigment-laden macrophages, extracellular green stringy material with histiocytes 2
- Pancreatic ascites: Amylase >1,000 mg/dL 1
- Chylous ascites: Triglycerides >200 mg/dL (often >1,000 mg/dL), white milky appearance 1
Critical Pitfall
Do NOT order routine cytology for suspected bacterial peritonitis—it is expensive, unnecessary, and will not change management. 1 The cell count with differential is the essential test, providing the PMN count that establishes the diagnosis. Reserve cytology exclusively for cases where malignant peritonitis is suspected based on clinical history and presentation.