What is the normal composition of ascitic fluid?

Normal Ascitic Fluid Composition

Normal ascitic fluid in cirrhotic patients is sterile, typically clear, and contains approximately 281 leukocytes/mm³ (mean), with 27% being polymorphonuclear cells (PMN). 1

Cell Count Parameters

• Total white blood cell count: 281 ± 25 cells/mm³ in uncomplicated cirrhotic ascites 1
• Polymorphonuclear cells (PMN): Approximately 27% of total leukocytes, which translates to roughly 75-80 PMN cells/mm³ 1
• Critical threshold: PMN count <250 cells/mm³ is considered normal and rules out spontaneous bacterial peritonitis (SBP) 2
• Red blood cells: Usually <1,000 cells/mm³ in cirrhotic ascites 2

Protein Composition

• Total protein concentration: Typically low in cirrhotic ascites, generally <2.5 g/dL 3
• Albumin concentration: Mean of 1.04 ± 0.73 g/dL in cirrhotic ascites 3
• Globulin concentration: Mean of 1.31 ± 0.80 g/dL 3
• Serum-Ascites Albumin Gradient (SAAG): ≥1.1 g/dL indicates portal hypertension-related ascites with 97% accuracy 4

Important caveat: The ascitic fluid protein concentration is not fixed and increases during diuresis, potentially rising from baseline 1.38 g/dL to 2.86 g/dL, which can confound interpretation if paracentesis is performed after significant diuretic therapy 5

Biochemical Parameters

• Glucose: Not useful for differential diagnosis in uncomplicated ascites 1
• Lactate dehydrogenase (LDH): Should be less than the upper limit of normal for serum in uncomplicated cirrhotic ascites 2
• Amylase: Normal levels; elevated amylase (typically >1,000 IU/L) suggests pancreatic ascites 2
• Specific gravity: Not useful for differential diagnosis 1

Microbiological Characteristics

• Culture: Sterile in normal ascitic fluid 1
• Gram stain: Negative for bacteria 1
• Culture technique: When infection is suspected, inoculation into blood culture bottles at bedside increases yield to approximately 80% compared to 50% with conventional methods 2

Distinguishing Features by SAAG and Protein

High SAAG (≥1.1 g/dL) + Low protein (<2.5 g/dL) = Typical cirrhotic ascites 4

High SAAG (≥1.1 g/dL) + High protein (>2.5 g/dL) = Cardiac ascites 4

• In cardiac cirrhosis specifically, ascitic fluid protein is significantly lower (mean 3.66 g/dL) compared to non-cirrhotic cardiac ascites (4.31 g/dL), likely due to decreased serum protein levels 6

Low SAAG (<1.1 g/dL) = Non-portal hypertension causes such as peritoneal carcinomatosis, tuberculous peritonitis, nephrotic syndrome, or pancreatic ascites 7, 4

Essential Initial Testing

The initial laboratory investigation should include 2:

• Ascitic fluid cell count with differential to calculate absolute PMN count
• Ascitic fluid total protein to assess infection risk and etiology
• Serum and ascitic fluid albumin to calculate SAAG
• Culture in blood culture bottles if infection is suspected

Clinical Pitfalls

• Diuresis effect: White blood cell concentration can increase from 289 cells/mm³ to 1,108 cells/mm³ during diuresis, though PMN percentage remains stable 5
• Multiple causes: Approximately 5% of patients have two or more causes of ascites simultaneously, and portal hypertension plus a second cause will still show SAAG ≥1.1 g/dL 4
• Bloody ascites: Occurs in about 2% of cirrhotics, with approximately 30% having underlying hepatocellular carcinoma 2
• CA-125 elevation: Elevated in ascites from any cause due to mesothelial cell pressure; not useful for differential diagnosis 2