What laboratory tests are recommended for a patient undergoing paracentesis?

Laboratory Tests for Paracentesis

For new-onset ascites, order cell count with differential, albumin (both serum and ascitic fluid to calculate SAAG), total protein, and bacterial culture in blood culture bottles at bedside. 1

Essential Tests for All Initial Paracenteses

Core Diagnostic Panel

• Cell count with differential: Identifies spontaneous bacterial peritonitis (SBP) when neutrophils ≥250 cells/mm³ 1
• Ascitic fluid albumin: Calculate serum-ascites albumin gradient (SAAG) by subtracting ascitic fluid albumin from serum albumin 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
• Bacterial culture: Inoculate 10-20 mL of ascitic fluid directly into blood culture bottles at bedside to increase yield from 50% to 80% 1
• Total protein: Helps identify high-risk patients for SBP (protein <1.5 g/dL) 1

Additional First-Line Tests

• Serum albumin: Required to calculate SAAG 1
• Ascitic fluid amylase: Order when pancreatic ascites is suspected 1

Tests for Serial Therapeutic Paracenteses

For asymptomatic outpatients undergoing repeat large-volume paracentesis, only cell count with differential is necessary. 1

• Bacterial culture is not required in asymptomatic patients undergoing serial procedures 1
• One study detected only 8 episodes of SBP in approximately 400 outpatient paracenteses over 2 years 1

Selective Testing Based on Clinical Suspicion

Cytology

• Order only when peritoneal carcinomatosis is suspected (history of breast, colon, gastric, or pancreatic cancer) 1
• Sensitivity is 96.7% when three 50 mL samples are sent fresh and processed immediately 1
• First sample detects 82.8% of cases 1
• Cytology is positive only in peritoneal carcinomatosis, not hepatocellular carcinoma 1

Mycobacterial Testing

• Order only for high-risk patients: recent immigration from endemic areas or AIDS 1
• Smear sensitivity is approximately 0%; culture sensitivity is approximately 50% 1
• Laparoscopy with biopsy is more accurate than fluid analysis for tuberculous peritonitis 1

Tests NOT Routinely Required

Pre-Procedure Coagulation Studies

Do not routinely measure prothrombin time/INR or platelet count before paracentesis, and do not transfuse blood products prophylactically. 1

• No hemorrhagic complications occurred in 1,100 procedures despite INR as high as 8.7 and platelets as low as 19,000/mm³ 1, 2
• Bleeding complications occur in only 0-2.7% of procedures 1
• Fresh frozen plasma or platelet transfusions before paracentesis are not recommended 1

Concurrent Blood Tests

Order these serum tests alongside paracentesis: 1

• Urea and electrolytes
• Liver function tests
• Prothrombin time (for prognostic purposes, not to guide procedure)
• Full blood count

Common Pitfalls to Avoid

• Do not send fluid in syringes or tubes to the laboratory for culture—this reduces sensitivity from 80% to 50% 1
• Do not order expensive tests (cytology, mycobacterial studies) routinely—reserve for high pretest probability 1
• Do not delay paracentesis for coagulopathy correction—complications remain rare even with severe coagulopathy 1, 2
• Do not omit cell count in serial paracenteses—SBP can occur in asymptomatic patients 1

Ultrasound Guidance Recommendation

Use ultrasound guidance when available to reduce adverse events, particularly bleeding complications. 1, 3

• Ultrasound guidance reduces bleeding risk by 68% after adjustment 1
• Helps identify optimal puncture site in left lower quadrant where ascites depth is greatest 1