Ascitic fluid analysis

Overview & Indications

• Diagnostic paracentesis is indicated for:
  • New-onset ascites of unknown etiology.
  • Suspected Spontaneous Bacterial Peritonitis (SBP) (e.g., fever, abdominal pain, altered mental status/HE, clinical worsening in a pt with pre-existing ascites).
  • Refractory ascites to guide therapeutic decisions.

Initial Evaluation

• Macroscopic Appearance:
  • Clear/straw-colored: Normal or uncomplicated cirrhosis.
  • Turbid/cloudy: Infection (SBP) or malignancy.
  • Bloody: Trauma (traumatic tap), malignancy, or ruptured ectopic pregnancy.
  • Milky: Chylous ascites (lymphatic obstruction).
• Routine Tests: Albumin, Total protein, Cell count w/ differential, and Culture (inoculated directly at bedside).

Serum-Ascites Albumin Gradient (SAAG)

• Calculation: $\text{SAAG}=\text{Serum Albumin}-\text{Ascitic Fluid Albumin}$.
• High SAAG ($\ge$ 1.1 g/dL): Indicates Portal Hypertension (PH).
  • High Protein ($\ge$ 2.5 g/dL): Cardiac ascites (CHF, constrictive pericarditis), early Budd-Chiari syndrome, or sinusoidal obstruction.
  • Low Protein (< 2.5 g/dL): Cirrhosis, late Budd-Chiari syndrome, or portal vein thrombosis.
• Low SAAG (< 1.1 g/dL): Indicates Non-Portal Hypertension.
  • High Protein ($\ge$ 2.5 g/dL): Peritoneal carcinomatosis, peritoneal tuberculosis (TB), fungal/bacterial peritonitis, or pancreatic ascites.
  • Low Protein (< 2.5 g/dL): Nephrotic syndrome (proteinuria depletes serum proteins).

Cell Count & Differential

• Spontaneous Bacterial Peritonitis (SBP):
  • Defined as PMN count $\ge$ 250/mm³ ($\ge 0.25\times 10^9$/L) in the absence of an intra-abdominal surgically treatable source. c
  • Bloody tap correction: Subtract 1 PMN for every 250 RBCs to obtain the true PMN count.
• Secondary Peritonitis:
  • PMN count is usually extremely high (often > 1,000/mm³).
  • Differentiated from SBP by:
    • Multimicrobial infection on Gram stain/culture (SBP is monomicrobial, usually E. coli or Klebsiella).
    • Runyon’s Criteria (at least 2 of the following): Glucose < 50 mg/dL, LDH > upper limit of normal for serum, Total protein > 1 g/dL.

Auxiliary Tests

• Amylase: Elevated (> 1,000 U/L) in pancreatic ascites (pancreatitis or ductal rupture).
• Triglycerides: $>200$ mg/dL (often > 500 mg/dL) in chylous ascites (lymphatic leak/obstruction, e.g., lymphoma, trauma).
• Cytology: Confirms peritoneal carcinomatosis (high sensitivity for primary peritoneal or ovarian malignancies; lower for GI metastases).
• Adenosine Deaminase (ADA): Elevated in peritoneal tuberculosis (useful where smear/culture sensitivity is low).

Clinical Management Decisions

• If SBP diagnosed (PMN $\ge$ 250/mm³):
  • Start Empiric Abx: 3rd generation cephalosporin (e.g., Ceftriaxone or Cefotaxime).
  • Give IV Albumin: On day 1 (1.5 g/kg) and day 3 (1.0 g/kg) to reduce the risk of hepatorenal syndrome (HRS) in pts with Cr > 1.0 mg/dL, BUN > 30 mg/dL, or Bilirubin > 4 mg/dL.
• If Secondary Peritonitis suspected:
  • Obtain abdominal CT to find perforation or abscess.
  • Add anaerobic coverage (e.g., Metronidazole) + immediate surgical consult.