When to Order Ascitic Fluid CEA
Order ascitic fluid CEA only when you suspect gut perforation into ascites (secondary bacterial peritonitis) in a patient with cirrhosis and an ascitic fluid PMN count ≥250 cells/mm³—not for routine evaluation of malignant ascites. 1
Primary Indication: Detecting Gut Perforation
The only guideline-endorsed indication for ascitic fluid CEA is to distinguish secondary bacterial peritonitis (gut perforation) from spontaneous bacterial peritonitis (SBP):
- Ascitic fluid CEA >5 ng/mL has 92% sensitivity and 88% specificity for detecting gut perforation into ascitic fluid 1
- This test should be ordered alongside ascitic fluid alkaline phosphatase (>240 U/L), total protein (>1 g/dL), LDH (>upper limit of normal for serum), and glucose (<50 mg/dL) when PMN count is ≥250 cells/mm³ 1
- The combination of CEA >5 ng/mL OR alkaline phosphatase >240 U/L identifies patients who need emergent imaging and surgical evaluation 1
Why CEA Should NOT Be Used for Malignant Ascites
Despite older research suggesting utility in detecting peritoneal carcinomatosis 2, 3, 4, current guidelines explicitly do not recommend CEA for diagnosing malignant ascites for these reasons:
- Cytology is the gold standard: Sensitivity reaches 96.7% when three 50-mL samples of fresh warm fluid are sent for immediate processing 1
- The first cytology sample alone detects 82.8% of peritoneal carcinomatosis cases 1
- CEA has poor sensitivity (43.1% pooled estimate) for malignancy-related ascites, though specificity is high (95.5%) 5
- Guidelines state that additional tests should only be ordered based on high pretest probability—cytology is ordered when peritoneal carcinomatosis is suspected (typically patients with known breast, colon, gastric, or pancreatic cancer) 1
Critical Pitfall to Avoid
Never order serum CA-125 in any patient with ascites. This marker is nonspecifically elevated in all patients with ascites from any cause (including men and women with prior oophorectomy), leading to unnecessary gynecologic referrals and potentially fatal surgeries when cirrhosis is ultimately found at laparotomy 1
Practical Algorithm for Ascitic Fluid Testing
Initial paracentesis in all patients with new ascites: 1, 6
- Cell count with differential
- Total protein
- Albumin (to calculate SAAG with simultaneous serum albumin)
- Culture in blood culture bottles at bedside if infection suspected
If PMN ≥250 cells/mm³ (suspected bacterial peritonitis): 1
- Add: Gram stain, total protein, LDH, glucose, CEA, and alkaline phosphatase
- If CEA >5 ng/mL or alkaline phosphatase >240 U/L → emergent imaging for gut perforation
If low SAAG (<1.1 g/dL) with clinical suspicion of malignancy: 1, 7
- Send cytology (50 mL fresh warm fluid, hand-carried for immediate processing)
- Do NOT order CEA for this indication
If high SAAG (≥1.1 g/dL) with lymphocytosis and TB risk factors: 7
- Order adenosine deaminase (ADA) ≥32-40 U/L (or ≥27 U/L in cirrhosis)
- Do NOT order acid-fast bacilli smear (0% sensitivity) or routine mycobacterial culture 1