What is the initial step in managing ascites, specifically regarding ascitic fluid analysis?

Initial Management of Ascites: Ascitic Fluid Analysis

Perform a diagnostic paracentesis immediately in all patients with new-onset ascites, and send the fluid for cell count with differential, total protein, and albumin to calculate the serum-ascites albumin gradient (SAAG). 1

Mandatory Initial Testing

The following tests must be performed on all patients with new-onset ascites:

• Cell count and differential - Essential to detect spontaneous bacterial peritonitis (SBP), which is diagnosed when neutrophils exceed 250 cells/mm³ 1
• Ascitic fluid albumin - Required to calculate SAAG, which differentiates portal hypertension from other causes with 97% accuracy 1, 2
• Serum albumin (same day) - Must be drawn simultaneously to calculate SAAG accurately 1, 2
• Ascitic fluid total protein - Identifies patients at high risk for SBP (protein <15 g/L) and helps distinguish cardiac ascites (protein >2.5 g/dL with high SAAG) from cirrhotic ascites 1, 3

Interpreting the SAAG

SAAG ≥1.1 g/dL indicates portal hypertension with 97% accuracy and predicts response to sodium restriction and diuretics 1, 2:

• High SAAG causes: Cirrhosis, cardiac ascites, Budd-Chiari syndrome, sinusoidal obstruction syndrome 3, 2
• Low SAAG causes (<1.1 g/dL): Peritoneal carcinomatosis, tuberculous peritonitis, pancreatic ascites, nephrotic syndrome 1, 2

Critical Diagnostic Pattern

When SAAG ≥1.1 g/dL AND protein >2.5 g/dL, this specifically indicates cardiac ascites rather than cirrhotic ascites (which typically has high SAAG but low protein) 3, 2

Bedside Culture Technique

Inoculate 10 mL of ascitic fluid directly into aerobic and anaerobic blood culture bottles at the bedside when infection is suspected, as this increases culture positivity rates 1

Conditional Testing Based on Clinical Suspicion

Order these additional tests only when specific diagnoses are suspected:

• Cytology - For suspected malignancy (yield varies 0-96.7% depending on tumor site) 1
• Amylase - For suspected pancreatic ascites (typically >1,000 IU/L or >6 times serum amylase) 1
• Adenosine deaminase - For suspected tuberculous peritonitis (levels <40 IU/L exclude TB with high accuracy) 1
• Creatinine and urea - For suspected urinary ascites from bladder or ureter injury (markedly elevated levels confirm urine) 1, 4
• CEA and alkaline phosphatase - For suspected secondary peritonitis from perforated viscus (CEA >5 ng/mL or alkaline phosphatase >240 U/L) 1

When to Perform Diagnostic Paracentesis

Beyond new-onset ascites, perform paracentesis urgently in hospitalized cirrhotic patients with:

• Any signs of infection - Fever, abdominal pain, systemic inflammation 1
• Gastrointestinal bleeding 1
• Hepatic encephalopathy 1
• Worsening renal or liver function 1
• Hypotension or shock 1
• Hospital admission (routine screening for SBP) 1

Common Pitfalls to Avoid

• Do not rely on fluid appearance alone - Both ascites and urine can appear clear or cloudy; creatinine level definitively distinguishes them 4
• Do not use CA125 - This marker is elevated by ascites from any cause and has no diagnostic value 1
• Recognize mixed ascites - Approximately 5% of patients have multiple causes contributing to fluid accumulation 1, 3
• Culture-negative neutrocytic ascites (PMN >250 cells/mm³ with negative culture) requires the same treatment as culture-positive SBP due to similar morbidity and mortality 1

Automated vs Manual Cell Counting

Either automated flow cytometry or manual microscopy can accurately determine the ascitic neutrophil count for SBP diagnosis 1