What labs should be ordered for diagnostic paracentesis?

Essential Laboratory Tests for Diagnostic Paracentesis

A diagnostic paracentesis should include neutrophil count, total protein, albumin, and bedside inoculation of ascitic fluid into blood culture bottles as the standard initial laboratory tests. 1

Core Laboratory Tests for All Diagnostic Paracenteses

• Cell count with differential - Essential to diagnose spontaneous bacterial peritonitis (SBP) with a neutrophil count >250 cells/mm³ as the diagnostic threshold 1
• Total protein concentration - Helps assess risk of SBP (values <1.5 g/dL indicate increased risk) and aids in differential diagnosis 1
• Albumin - Required to calculate the serum-ascites albumin gradient (SAAG), which differentiates portal hypertension-related ascites (SAAG ≥1.1 g/dL) from other causes with 97% accuracy 1
• Bacterial culture - Bedside inoculation of 10 mL of ascitic fluid into blood culture bottles significantly improves culture yield (from approximately 50% to >80-90%) 1

Additional Tests Based on Clinical Suspicion

• Cytology - Order when malignancy is suspected; sensitivity approaches 96.7% when three samples are processed promptly 1
• Glucose and LDH - Order when secondary bacterial peritonitis is suspected; glucose <50 mg/dL or ascitic LDH higher than serum LDH suggests secondary peritonitis 1
• Amylase - Order when pancreatic ascites is suspected; typically >1,000 mg/dL in pancreatic ascites 1
• Adenosine deaminase - Order when tuberculosis is suspected; levels <40 IU/mL help exclude TB 1
• CEA or alkaline phosphatase - Consider when perforated viscus is suspected; elevated CEA (>5 ng/mL) or alkaline phosphatase (>240 U/L) can help diagnose secondary peritonitis 1

Timing and Clinical Scenarios

• Perform diagnostic paracentesis in all patients with new-onset grade 2 or 3 ascites 1
• Always perform paracentesis in patients hospitalized for worsening ascites or any complication of cirrhosis 1
• Urgently perform paracentesis in patients with signs of infection, GI bleeding, shock, fever, gastrointestinal symptoms, hepatic encephalopathy, or worsening liver/renal function 1

Interpretation Algorithm

1. First, assess for infection:

   • Neutrophil count >250 cells/mm³ indicates SBP 1
   • Culture results guide antibiotic therapy 1

2. Second, determine the cause of ascites:

   • Calculate SAAG = serum albumin - ascitic fluid albumin
   • SAAG ≥1.1 g/dL indicates portal hypertension (97% accuracy) 1
   • SAAG <1.1 g/dL suggests non-portal hypertension causes (malignancy, TB, pancreatic disease) 1

3. Third, assess additional parameters based on clinical context:

   • Low ascitic protein (<1.5 g/dL) indicates increased risk for SBP 1
   • Abnormal glucose, LDH, amylase, or CEA may indicate specific diagnoses 1

Common Pitfalls to Avoid

• Do not order CA-125 for ascites evaluation as it is nonspecific and elevated in ascites of any cause 1
• Do not routinely order cytology, TB cultures, or amylase unless there is specific clinical suspicion 1
• Do not delay paracentesis in patients with suspected SBP, as immediate diagnosis and treatment are critical 1
• Do not wait for coagulation test results before performing paracentesis, as the procedure is safe even with abnormal coagulation parameters 1, 2