Can ascites (accumulation of fluid in the peritoneal cavity) be missed on a computed tomography (CT) scan in adults with a history of liver disease, cancer, or other conditions?

Can Ascites Be Missed on CT Scan?

Yes, ascites can be missed on CT scan, particularly in obese patients or when small volumes are present, which is why ultrasound is often preferred for initial detection and diagnostic paracentesis remains the gold standard for diagnosis. 1

Detection Limitations of CT

While CT has high accuracy (93%) for detecting ascites when present in moderate to large volumes 2, several clinical scenarios can lead to missed or difficult detection:

Small Volume Ascites

• Physical examination can only detect ascites when approximately 1500 mL of fluid is present (based on flank dullness), and imaging modalities including CT may similarly miss smaller volumes 1
• CT is generally excellent for moderate to large ascites but may not reliably detect trace amounts 2

Obese Patients

• The physical examination for detecting ascites in obese patients is problematic, and an abdominal ultrasound may be required to determine with certainty if fluid is present 1
• This same limitation applies to CT imaging, where body habitus can obscure small fluid collections 1

Loculated or Atypical Fluid Distribution

• Fluid confined to the lesser sac or loculated collections may be overlooked if not specifically evaluated 3
• Benign transudative ascites (cirrhosis, heart failure) typically shows large greater sac collections with minimal lesser sac fluid, while diseases of organs bordering the lesser sac show the opposite pattern 3
• Malignant ascites often shows concordant fluid volumes in both spaces 3

Why Ultrasound and Paracentesis Are Preferred

Ultrasound Advantages

• Ultrasound is more sensitive for detecting small volumes of ascites and is the recommended imaging modality when clinical suspicion exists but physical examination is equivocal 1
• Particularly valuable in obese patients where both physical examination and CT may be limited 1

Paracentesis as Gold Standard

• Abdominal paracentesis with appropriate ascitic fluid analysis is the most rapid and cost-effective method of diagnosing ascites and determining its cause 1, 4
• Diagnostic paracentesis should be performed in all patients with new-onset Grade 2 or 3 ascites 1
• Physical examination alone is insufficient for diagnosis, and delaying paracentesis can miss the etiology 4

Clinical Pitfalls to Avoid

Relying Solely on Imaging

• Do not rely on CT alone when clinical suspicion for ascites is high but imaging is negative—proceed to ultrasound or diagnostic paracentesis 1, 4
• In patients with slowly enlarging abdomen over months to years, obesity is more likely than ascites (which typically presents over weeks) 1

Missing Mimics

• Giant cysts or pseudocysts can rarely mimic ascites on physical examination, but imaging usually provides the correct diagnosis 1
• Consider alternative diagnoses when paracentesis produces fluid with unusual characteristics 1

Overlooking Mixed Etiologies

• Approximately 5% of patients have two or more causes of ascites formation (e.g., cirrhosis plus peritoneal carcinomatosis or tuberculosis) 1, 4
• This "mixed ascites" requires comprehensive evaluation beyond imaging alone 4

Recommended Diagnostic Approach

When ascites is suspected clinically:

1. Perform physical examination looking for flank dullness and shifting dullness (83% sensitivity, 56% specificity) 1

2. If physical examination is equivocal or patient is obese, obtain abdominal ultrasound rather than CT as the initial imaging study 1

3. Proceed directly to diagnostic paracentesis regardless of imaging findings when clinical suspicion is high 1, 4

4. Reserve CT for evaluating complications (hepatocellular carcinoma, portal vein thrombosis, hepatic vein thrombosis) rather than initial ascites detection 1