If a CT scan shows an abnormality in a generally healthy adult with new‑onset ascites, do I need additional tests?

Yes, you need additional tests beyond the CT scan—specifically, diagnostic paracentesis with ascitic fluid analysis is mandatory.

If a CT scan shows an abnormality in a generally healthy adult with new-onset ascites, you must perform an immediate diagnostic paracentesis with ascitic fluid analysis; imaging alone cannot determine the cause of ascites or detect life-threatening complications such as spontaneous bacterial peritonitis. 1, 2

Why CT Findings Are Insufficient

• CT cannot differentiate the etiology of ascites. While imaging confirms fluid presence and may reveal structural abnormalities (liver lesions, lymphadenopathy, masses), it cannot distinguish between portal hypertension, malignancy, infection, cardiac causes, tuberculosis, or pancreatic disease. 3, 4

• CT cannot detect spontaneous bacterial peritonitis (SBP). Approximately 15% of patients with ascites have SBP at presentation, a life-threatening infection that requires immediate antibiotic therapy. Only ascitic fluid analysis with a neutrophil count can diagnose this. 1, 2

• Imaging has limited sensitivity for small fluid volumes. CT may miss ascites <100 mL, whereas ultrasound detects volumes as small as 100 mL and physical examination requires ≥1,500 mL. 5, 6

Mandatory Immediate Testing: Diagnostic Paracentesis

Perform paracentesis at the bedside as soon as new-onset ascites is identified. This is the fastest, most cost-effective method to determine etiology and can be done safely even with coagulopathy (complications occur in only ~1% of procedures). 1, 2

Core Ascitic Fluid Tests (Required for Every Patient)

1. Cell count with differential – A neutrophil count ≥250 cells/mm³ defines SBP and mandates immediate empirical antibiotics (cefotaxime 2 g IV every 8 hours) without waiting for culture results. 1, 2

2. Ascitic fluid albumin AND simultaneous serum albumin – Calculate the serum-ascites albumin gradient (SAAG):

   • SAAG ≥1.1 g/dL indicates portal hypertension with 97% accuracy (cirrhosis, massive liver metastases, cardiac failure, Budd-Chiari syndrome). 1, 4
   • SAAG <1.1 g/dL suggests non-portal hypertensive causes (peritoneal carcinomatosis, tuberculosis, pancreatic ascites, nephrotic syndrome). 1, 4

3. Ascitic fluid total protein – Levels <1.5 g/dL identify high risk for developing SBP and guide prophylactic antibiotic decisions. 1, 2

4. Bacterial culture in blood culture bottles – Inoculate ≥10 mL of ascitic fluid into aerobic and anaerobic blood culture bottles at the bedside before any antibiotics are given; this increases culture yield from ~50% to 80-90%. 1, 2

Additional Tests Based on CT Findings & Clinical Context

If CT Shows Mesenteric Lymphadenopathy or Masses

• Cytology – Send three fresh 50-mL samples for cytologic examination; sensitivity is 96.7% for peritoneal carcinomatosis when three samples are processed (first sample alone is positive in 82.8%). 2

• Adenosine deaminase (ADA) – Order when tuberculous peritonitis is suspected (endemic regions, HIV, lymphocyte-predominant fluid). 2, 7, 3

If CT Shows Liver Abnormalities

• Brain natriuretic peptide (BNP) – Median pro-BNP is ~6,100 pg/mL in cardiac ascites versus ~166 pg/mL in cirrhotic ascites; this helps distinguish alcoholic cardiomyopathy from alcoholic cirrhosis. 2, 7, 8

• Ascitic fluid glucose, LDH, and Gram stain – If secondary bacterial peritonitis (bowel perforation) is suspected: glucose <50 mg/dL, LDH higher than serum LDH, or multiple organisms on Gram stain indicate surgical emergency. 1, 2

• Ascitic fluid CEA >5 ng/mL or alkaline phosphatase >240 U/L – Suggests gut perforation into ascitic fluid. 1, 2

If CT Shows Pancreatic Abnormalities

• Ascitic fluid amylase – Levels >1,000 U/L or >6× serum amylase support pancreatic ascites. 2, 3

Safety Considerations for Paracentesis

• Do NOT withhold paracentesis for coagulopathy. Abnormal INR (up to 8.7) or thrombocytopenia (as low as 19 × 10³/µL) are not contraindications. Major bleeding occurs in only ~1% of procedures, primarily minor abdominal wall hematomas. 1, 2

• Do NOT give prophylactic fresh frozen plasma or platelets before paracentesis; this is not supported by evidence. 1, 2

• Absolute contraindications are limited to clinically evident disseminated intravascular coagulation or active fibrinolysis (occurs in <1 per 1,000 procedures). 1, 2

Common Pitfalls to Avoid

• Never rely on CT alone to determine ascites etiology. Imaging cannot calculate SAAG, detect infection, or guide specific therapy. 8, 3, 4

• Never delay paracentesis to "correct" coagulation parameters. This delays diagnosis of SBP, which carries high mortality if untreated. 1, 2

• Always obtain cell count and culture even when malignancy is suspected on CT, because SBP can coexist with malignant ascites. 8

• Do not order expensive ancillary tests (cytology, ADA, tumor markers) reflexively. Reserve these for situations with high pretest probability based on CT findings and clinical context. 2

Algorithmic Approach

1. CT shows ascites + abnormality → Perform immediate diagnostic paracentesis 1, 2

2. Send core panel: cell count, albumin (fluid + serum), total protein, culture in blood culture bottles 1, 2

3. Interpret neutrophil count:

   • ≥250 cells/mm³ → Start cefotaxime 2 g IV q8h immediately + albumin 1.5 g/kg IV 1, 2
   • <250 cells/mm³ → Proceed to SAAG interpretation 1

4. Calculate SAAG:

   • ≥1.1 g/dL → Portal hypertension; evaluate liver disease, cardiac function, hepatic vein patency 1, 4
   • <1.1 g/dL → Order cytology (if CT shows masses/nodes), ADA (if TB suspected), amylase (if pancreatic disease) 2, 3

5. If CT shows specific findings:

   • Lymphadenopathy/masses → Add cytology + ADA 2, 7
   • Liver lesions → Add BNP to exclude cardiac cause 2, 8
   • Pancreatic abnormality → Add amylase 2, 3