How should ascitic fluid be classified?

Classification of Ascitic Fluid

Ascitic fluid should be classified primarily using the serum-ascites albumin gradient (SAAG), which distinguishes portal hypertensive from non-portal hypertensive causes with 97% accuracy, followed by cell count analysis to identify infection. 1, 2

Primary Classification: SAAG Calculation

The SAAG is calculated by subtracting the ascitic fluid albumin concentration from the serum albumin concentration obtained on the same day 1:

• SAAG ≥1.1 g/dL indicates portal hypertension with 97% accuracy 1, 2

  • Causes include: cirrhosis, cardiac ascites, Budd-Chiari syndrome, sinusoidal obstruction syndrome 1

• SAAG <1.1 g/dL indicates non-portal hypertensive ascites 1, 2

  • Causes include: peritoneal carcinomatosis, tuberculous peritonitis, pancreatic ascites 2, 3

This SAAG-based approach has replaced the outdated transudate/exudate classification based on total protein, which is no longer recommended 3.

Secondary Classification: Infection Status

The ascitic fluid polymorphonuclear (PMN) cell count is the most accurate test for diagnosing spontaneous bacterial peritonitis (SBP), with 96% diagnostic accuracy 2:

• PMN count ≥250 cells/mm³ indicates SBP and requires immediate empiric antibiotic therapy 1
• PMN count >500 cells/mm³ has even greater diagnostic accuracy 2
• **PMN count <250 cells/mm³ with signs of infection** (fever >100°F, abdominal pain/tenderness) still warrants empiric treatment 1

Additional Classification Parameters

For Suspected Malignancy (when SAAG <1.1 g/dL):

• Ascitic fluid lactate dehydrogenase (LDH) and cholesterol are most useful for diagnosing malignant ascites 4, 3
• Cytology should be performed 3
• Tumor markers (CEA, VEGF) can provide additional diagnostic information 1, 3

For Suspected Secondary Peritonitis (when PMN ≥250 cells/mm³):

Order additional tests to distinguish from SBP 1:

• At least 2 of the following suggest gut perforation: total protein >1 g/dL, LDH greater than upper limit of normal for serum, glucose <50 mg/dL 1
• Multiple organisms on Gram stain and culture 1
• CEA >5 ng/mL or alkaline phosphatase >240 U/L has 92% sensitivity and 88% specificity for gut perforation 1

For Suspected Tuberculous Peritonitis:

• Ascitic fluid lymphocytosis is typical 1
• Adenosine deaminase (ADA) >32-40 U/L has 100% sensitivity and 96.6-100% specificity in non-cirrhotic patients 1, 3
• Glucose levels and ascitic fluid/blood glucose ratio are useful for diagnosis 4, 3
• AFB smear (0-86% sensitivity) and culture (20-83% sensitivity) should be performed 1

For Cardiac Ascites:

• High SAAG (≥1.1 g/dL) combined with high protein (>2.5 g/dL) specifically indicates cardiac ascites 5
• This distinguishes it from cirrhotic ascites, which has high SAAG but low protein (<2.5 g/dL) 5

Practical Algorithm for Initial Analysis

On every diagnostic paracentesis, obtain 1, 2:

1. Cell count with differential (most critical for infection diagnosis) 2
2. Albumin (for SAAG calculation; obtain simultaneous serum albumin) 1, 2
3. Total protein (helps distinguish cardiac from cirrhotic ascites) 1
4. Inoculate blood culture bottles at bedside for bacterial culture 6

Additional tests should be ordered selectively based on clinical context rather than routinely 4, 7:

• If PMN ≥250 cells/mm³: add Gram stain, LDH, glucose, total protein 1
• If SAAG <1.1 g/dL: add cytology, LDH, cholesterol 4, 3
• If tuberculous peritonitis suspected: add ADA, AFB smear/culture, glucose 1, 3

Critical Pitfalls to Avoid

• Do not use total protein alone to classify ascites as transudate versus exudate—this approach is outdated and inaccurate 3
• Ascitic fluid pH and lactate are not reliable for SBP diagnosis due to false positives in malignant ascites, tuberculous peritonitis, and pancreatic ascites 2
• Approximately 5% of patients have "mixed ascites" with multiple contributing causes, requiring comprehensive evaluation 5
• Delayed transport affects cell counts and culture results—specimens must be sent to the laboratory immediately 6