What is the approach to diagnosing ascites?

Diagnostic Approach to Ascites

Abdominal paracentesis with ascitic fluid analysis is the most rapid and cost-effective method for diagnosing the cause of ascites and should be performed in all patients with new-onset Grade 2 or 3 ascites. 1, 2

Initial Clinical Assessment

History

• Question patients about risk factors for liver disease (alcohol use, viral hepatitis, metabolic syndrome) 1
• Assess for past history of malignancy, heart failure, renal disease, or tuberculosis 1
• Determine duration of abdominal distension: ascites typically develops over weeks, whereas obesity develops over months to years 1
• Screen for symptoms suggesting portal hypertension versus cardiac causes 1

Physical Examination

• Percuss the flanks first when abdominal swelling is observed 2
• Test for shifting dullness: this is the most sensitive physical examination technique with 83% sensitivity and 56% specificity 1, 2
• Approximately 1,500 mL of fluid must be present before flank dullness can be detected 1, 2
• If no flank dullness is present, the patient has less than 10% chance of having ascites 1, 2
• Assess for jugular venous distension to distinguish cardiac ascites (present) from cirrhotic ascites (absent) 1
• The fluid wave test is more specific but less sensitive and more cumbersome than shifting dullness 2

Confirmatory Imaging

• Abdominal ultrasound can detect ascites when volume exceeds 100 mL and should be used when physical examination is inconclusive, particularly in obese patients 1, 2
• Ultrasound should also evaluate liver appearance, splenomegaly, portal vein patency, hepatic vein patency, pancreas, and lymph nodes 1

Abdominal Paracentesis Technique

Indications

• Perform diagnostic paracentesis in all patients with new-onset Grade 2 or 3 ascites 2
• Perform surveillance paracentesis on hospital admission due to high prevalence (approximately 15%) of spontaneous bacterial peritonitis 1

Procedure Details

• Site: approximately 15 cm lateral to the umbilicus in the left or right lower quadrant 1
• Avoid the inferior and superior epigastric arteries that run lateral to the umbilicus toward the mid-inguinal point 1
• Avoid enlarged liver or spleen 1
• Withdraw 10-20 mL for diagnostic purposes 1
• Coagulopathy is NOT a contraindication: complications occur in only 1% of patients (abdominal wall hematomas), with serious complications (hemoperitoneum, bowel perforation) occurring in less than 1/1000 procedures 1, 2
• Fresh frozen plasma is not routinely indicated before paracentesis despite abnormal coagulation tests 1
• Consider platelet transfusion only if platelet count is severely low (<40,000/μL) 1

Essential Ascitic Fluid Analysis

Mandatory Tests

• Serum-ascites albumin gradient (SAAG): the most useful test to identify portal hypertension-related ascites 2
  • SAAG ≥1.1 g/dL indicates portal hypertension with 97% accuracy 3
  • SAAG <1.1 g/dL suggests non-portal hypertensive causes 2
• Ascitic fluid neutrophil count: screen for spontaneous bacterial peritonitis (present in approximately 15% of cirrhotic patients on admission) 1
• Ascitic fluid culture: inoculate into two blood culture bottles at bedside 1
• Ascitic fluid total protein concentration 1, 2

Optional Tests Based on Clinical Suspicion

• Cytology: when malignancy is suspected 1
• Amylase: when pancreatic ascites is suspected 1
• Triglyceride level: when chylous ascites is suspected (white, milky appearance); level >200 mg/dL confirms diagnosis 3
• Adenosine deaminase (ADA): when tuberculous peritonitis is suspected 4

Blood Tests

• Urea and electrolytes 1
• Liver function tests 1
• Prothrombin time 1
• Full blood count 1
• Brain natriuretic peptide (BNP) or pro-BNP: to distinguish cardiac ascites (median pro-BNP 6,100 pg/mL) from cirrhotic ascites (median pro-BNP 166 pg/mL) 1, 2

Differential Diagnosis Framework

Portal Hypertension-Related (SAAG ≥1.1 g/dL)

• Cirrhosis (75-85% of all ascites cases) 2
• Alcoholic hepatitis 1
• Acute liver failure 1
• Budd-Chiari syndrome 1
• Sinusoidal obstruction syndrome 1

Non-Portal Hypertension (SAAG <1.1 g/dL)

• Peritoneal carcinomatosis 1
• Tuberculous peritonitis 1
• Pancreatic ascites 1
• Nephrotic syndrome 1
• Postoperative lymphatic leak 1

Critical Pitfalls to Avoid

• Do not assume alcoholic patients have alcoholic liver disease: exclude other causes of ascites 1
• Physical examination alone may miss small volumes: less than 1,500 mL of ascites may not be detectable on examination 2
• Do not rely solely on fluid appearance: turbid fluid can result from infection or tumor cells, not just chylous ascites 3
• Do not delay paracentesis due to coagulopathy: routine coagulation tests do not reflect bleeding risk in cirrhosis, and prophylactic transfusions may cause more harm than benefit 1