Ascites Evaluation
Overview
Ascites is the pathological accumulation of free fluid within the peritoneal cavity, most commonly from portal hypertension in liver cirrhosis (75% of cases). Systematic diagnostic evaluation using ultrasound and paracentesis identifies the aetiology and guides management.
The SAAG: Key Discriminator
SAAG = Serum Albumin - Ascitic fluid Albumin. Calculated from the same blood draw and ascitic tap specimen:
- SAAG >=1.1 g/dL (High gradient = transudate): Caused by high portal pressure. Causes: Cirrhosis, Budd-Chiari syndrome, right heart failure, constrictive pericarditis.
- SAAG <1.1 g/dL (Low gradient = exudate): Caused by peritoneal pathology. Causes: Peritoneal carcinomatosis, TB peritonitis, pancreatitis, nephrotic syndrome.
Ultrasound Features
- Anechoic free-flowing fluid dependent in Morrison's pouch (right subhepatic) and the Pouch of Douglas.
- Complex fluid with echoes, septations, or debris suggests malignancy, infection, or haemorrhage.
- Peritoneal nodules/thickening on ultrasound or CT raises suspicion for peritoneal carcinomatosis or TB.
WarningSpontaneous Bacterial Peritonitis (SBP)
SBP occurs in up to 25% of hospitalised cirrhotic patients without an obvious source of infection. Diagnosis: ascitic fluid PMN count >250 cells/mm3. Common organisms: E. coli, Klebsiella. Treat empirically with IV third-generation cephalosporin. Without treatment, mortality approaches 50%.
High Yield Facts
LightbulbFRCR / MD Prep Pearl
Ultrasound can detect as little as 100 mL of ascites in skilled hands. The right lateral decubitus position increases sensitivity. Liver contour nodularity on ultrasound (irregular, bumpy surface) suggests underlying cirrhosis. CT/PET-CT is used to identify the primary tumour in malignant ascites.