Which laboratory studies should be obtained from ascitic fluid and serum after a diagnostic or therapeutic paracentesis?

Laboratory Studies After Paracentesis

Order ascitic fluid cell count with differential, ascitic fluid albumin, ascitic fluid total protein, and simultaneous serum albumin on every paracentesis to calculate the serum-ascites albumin gradient (SAAG), along with bedside inoculation of 10 mL ascitic fluid into blood culture bottles. 1

Essential Ascitic Fluid Tests (All Patients)

Core Diagnostic Panel

• Cell count with differential and absolute neutrophil count: A neutrophil count >250 cells/mm³ diagnoses spontaneous bacterial peritonitis (SBP) with 96% accuracy, making this the single most important test for detecting infection 1, 2

• Ascitic fluid albumin: Required to calculate the SAAG, which determines whether ascites is due to portal hypertension 1

• Ascitic fluid total protein: Concentrations <15 g/L (1.5 g/dL) identify patients at high risk for developing SBP and may guide prophylaxis decisions 1

• Bedside inoculation into blood culture bottles: Inoculate 10 mL of ascitic fluid directly into aerobic and anaerobic blood culture bottles at the bedside to maximize bacterial recovery (increases culture yield from 77% to 100%) 1, 3

Required Serum Tests

• Serum albumin: Must be drawn on the same day as paracentesis to calculate the SAAG 1

  • SAAG ≥1.1 g/dL indicates portal hypertension with 97% accuracy 1
  • SAAG <1.1 g/dL suggests non-portal hypertensive causes (malignancy, tuberculosis, nephrotic syndrome) 1, 2

• Liver function tests, renal function (creatinine, BUN), and serum electrolytes: Essential baseline assessment for all patients with ascites 1

Optional Tests (Only When Clinically Indicated)

Order these additional tests only when specific diagnoses are suspected based on clinical presentation or initial results:

• Ascitic fluid amylase: Order when pancreatic ascites is suspected (abdominal pain, history of pancreatitis) 1

• Ascitic fluid cytology: Request when malignancy-related ascites is suspected (bloody ascites, known cancer, weight loss) 1

• Acid-fast bacilli (AFB) smear, culture, and adenosine deaminase (ADA): Order when tuberculous peritonitis is suspected (fever, lymphocytic ascites, endemic area) 1

  • ADA >32-40 U/L has 100% sensitivity and 96.6-100% specificity for tuberculous peritonitis in non-cirrhotic patients 1

• Ascitic fluid triglycerides: Order if fluid appears milky white to diagnose chylous ascites (triglycerides >200 mg/dL, often >1,000 mg/dL) 1

• Ascitic fluid glucose: May help distinguish secondary bacterial peritonitis from SBP when very low 4

Common Pitfalls to Avoid

• Do not delay culture inoculation: Bedside inoculation into blood culture bottles is critical—delayed inoculation significantly reduces diagnostic yield 1, 3

• Do not skip the SAAG calculation in cirrhotic patients: Even in known cirrhosis, 38% of patients with low SAAG have an identifiable alternative cause (SBP, malignancy, nephrotic syndrome), and 73% will convert to high SAAG on repeat paracentesis 5

• Do not order unnecessary tests routinely: Amylase, cytology, AFB studies, and tumor markers should be reserved for specific clinical suspicions, not ordered reflexively 1

• Do not use ascitic fluid pH or lactate as primary SBP screening: While pH <7.35 or blood-ascitic fluid pH gradient ≥0.10 suggest SBP, these tests have lower specificity than neutrophil count due to false positives in malignant ascites, tuberculous peritonitis, and pancreatic ascites 2, 3

• Do not forget simultaneous serum albumin: The SAAG cannot be calculated without a same-day serum albumin level 1

Interpretation Algorithm

1. First, check neutrophil count: >250 cells/mm³ = start empiric antibiotics immediately for SBP 1

2. Second, calculate SAAG:

   • ≥1.1 g/dL → portal hypertension (cirrhosis, cardiac ascites, Budd-Chiari) 1
   • <1.1 g/dL → consider malignancy, tuberculosis, nephrotic syndrome, pancreatic disease 1, 2

3. Third, assess total protein: <15 g/L identifies high SBP risk and may warrant prophylactic antibiotics in select patients 1

4. Finally, review culture results at 48 hours: Positive cultures confirm infected ascites and guide antibiotic adjustment 1, 3