What tests should be sent for analysis when tapping ascites for the first time?

Essential Tests for First-Time Ascites Tapping

When performing a diagnostic paracentesis for ascites for the first time, the essential laboratory tests should include ascitic fluid cell count with differential, ascitic fluid total protein, ascitic fluid albumin (for SAAG calculation), and bacterial culture in blood culture bottles if infection is suspected. 1

Core Tests for All Patients with New-Onset Ascites

Mandatory Tests:

• Cell count and differential - Essential for detecting spontaneous bacterial peritonitis (SBP)
• Ascitic fluid albumin - For calculating serum-ascites albumin gradient (SAAG)
• Ascitic fluid total protein - Helps differentiate between causes of high SAAG ascites
• Serum albumin (drawn same day) - Required to calculate SAAG

When Infection is Suspected:

• Bacterial culture - Should be inoculated at bedside directly into blood culture bottles (not in tubes or syringes sent to the lab) to increase yield from ~50% to ~80% 1
• Culture is indicated when patient presents with:
  • Fever
  • Abdominal pain
  • Unexplained encephalopathy
  • Hypotension
  • Renal insufficiency
  • Gastrointestinal bleeding

Additional Tests Based on Clinical Suspicion

For Suspected Malignancy:

• Cytology - Most useful when peritoneal carcinomatosis is suspected; sensitivity is 82.8% for first sample, 96.7% if three samples are processed 1

For Suspected Secondary Peritonitis:

• Glucose - Level <50 mg/dL suggests secondary peritonitis
• LDH - Ascitic fluid LDH higher than serum LDH suggests secondary peritonitis
• CEA - >5 ng/mL suggests gut perforation
• Alkaline phosphatase - >240 U/L suggests gut perforation 1

For Suspected Tuberculous Peritonitis:

• Adenosine deaminase (ADA) - Elevated in tuberculous peritonitis
• Mycobacterial culture - Only ~50% sensitive; consider only in high-risk patients 1

For Suspected Pancreatic Ascites:

• Amylase - Typically >1,000 mg/dL in pancreatic ascites 1

For Suspected Urinary Ascites:

• Urea and creatinine - Elevated in urinary ascites from bladder/ureter injury 1

Interpretation of Results

SAAG Interpretation:

• SAAG ≥1.1 g/dL: Indicates portal hypertension (97% accuracy)

  • Common causes: Cirrhosis, alcoholic hepatitis, heart failure, Budd-Chiari syndrome
  • Usually responds to sodium restriction and diuretics

• SAAG <1.1 g/dL: Non-portal hypertension causes

  • Common causes: Peritoneal carcinomatosis, tuberculosis, nephrotic syndrome
  • Usually does not respond to sodium restriction and diuretics

Total Protein Interpretation:

• High SAAG, high protein (>2.5 g/dL): Suggests cardiac ascites
• High SAAG, low protein (<2.5 g/dL): Suggests cirrhotic ascites 2

Important Caveats

• Avoid CA125 testing - This test is non-specific and elevated in all types of ascites; may lead to unnecessary gynecologic procedures 1, 2

• No need for prophylactic fresh frozen plasma or platelets - Paracentesis can be safely performed in patients with coagulopathy; bleeding complications are rare 1

• Consider mixed ascites - Approximately 5% of patients have two or more causes of ascites formation (e.g., cirrhosis plus peritoneal carcinomatosis) 1

• For serial therapeutic paracenteses - Only cell count and differential are necessary; bacterial culture is not needed in asymptomatic patients 1

By following this systematic approach to ascitic fluid analysis, you can efficiently determine the etiology of ascites and detect complications like spontaneous bacterial peritonitis, which is critical for appropriate management and improving patient outcomes.