What are the causes of ascites presenting as excess fluid at the lower abdomen in an otherwise healthy adult?

Causes of Excessive Fluid at the Base of the Abdomen (Ascites)

Cirrhosis accounts for 75-85% of all ascites cases, making it by far the most common cause, followed by malignancy (9-10%), heart failure, tuberculosis, and nephrotic syndrome. 1

Primary Etiologies by Frequency

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

• Cirrhosis is the dominant cause, with alcoholic liver disease being particularly important because it is highly reversible with abstinence (75% 3-year survival if alcohol is stopped versus 0% if continued) 1
• Cardiac ascites from heart failure is distinguished by elevated jugular venous distention (absent in cirrhosis) and markedly elevated pro-brain natriuretic peptide levels (median 6100 pg/mL versus 166 pg/mL in cirrhosis) 1
• Vascular causes include Budd-Chiari syndrome and sinusoidal obstruction syndrome 1
• Portal vein thrombosis can cause portal hypertension without underlying cirrhosis 2

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

• Peritoneal carcinomatosis accounts for 9-10% of cases, most commonly from breast, colon, gastric, or pancreatic primaries 1
• Tuberculous peritonitis occurs in 10-12% of cases, particularly in recent immigrants from endemic areas or HIV/AIDS patients 1
• Nephrotic syndrome causes ascites through severe hypoalbuminemia 1
• Pancreatic ascites from pancreatitis (ascitic amylase typically >1,000 mg/dL) 3

Mixed Ascites (Multiple Simultaneous Causes)

Approximately 5% of patients have two or more causes simultaneously, typically cirrhosis plus peritoneal carcinomatosis or tuberculosis 3, 1. In some cases, the sum of predisposing factors (heart failure, diabetic nephropathy, and cirrhosis) leads to fluid retention when each individual factor alone would be insufficient 3.

Essential Diagnostic Algorithm

Step 1: Perform Diagnostic Paracentesis Immediately

Diagnostic paracentesis is mandatory for all new-onset Grade 2 or 3 ascites, hospitalization for worsening ascites, or any complication including fever, abdominal pain, gastrointestinal bleeding, hepatic encephalopathy, hypotension, or renal insufficiency. 2 Each hour of delay increases in-hospital mortality by 3.3% in patients with suspected spontaneous bacterial peritonitis 2.

Step 2: Order Essential Ascitic Fluid Tests

• Cell count with differential (neutrophil count >250 cells/mm³ confirms spontaneous bacterial peritonitis requiring immediate empiric antibiotics) 2
• Albumin (for SAAG calculation) 2
• Total protein concentration 2
• Culture in blood culture bottles at bedside (if infection suspected) 3

Step 3: Calculate SAAG with Simultaneous Serum Albumin

SAAG = Serum Albumin - Ascitic Fluid Albumin

• SAAG ≥1.1 g/dL indicates portal hypertension with 97% accuracy (cirrhosis, cardiac failure, portal vein thrombosis) 3, 2
• SAAG <1.1 g/dL indicates non-portal hypertension causes (peritoneal carcinomatosis, tuberculosis, pancreatitis, nephrotic syndrome) 2

Step 4: Additional Testing Based on Clinical Suspicion

For suspected peritoneal carcinomatosis:

• Cytology (96.7% sensitivity if three 50-mL samples of fresh warm fluid are hand-carried to lab for immediate processing; first sample positive in 82.8%) 3, 1
• History of breast, colon, gastric, or pancreatic primary carcinoma 3

For suspected tuberculous peritonitis:

• Adenosine deaminase (ADA) with cut-off value of 27-32 U/L (sensitivity 91.7-100%, specificity 92-93.3%) 3
• Laparoscopy with biopsy and mycobacterial culture of tubercles is the gold standard (fluid culture sensitivity only 50%, smear sensitivity approximately 0%) 1, 4
• High-risk patients include recent immigrants from endemic areas or HIV/AIDS 3

For suspected secondary peritonitis (perforated viscus):

• Ascitic glucose <50 mg/dL or ascitic LDH higher than serum LDH 3
• CEA >5 ng/mL or alkaline phosphatase >240 U/L 3

For suspected pancreatic ascites:

• Ascitic amylase level (typically >1,000 mg/dL) 3

Critical Pitfalls to Avoid

• Do not rely on physical examination alone in obese patients—proceed directly to ultrasound (physical exam unreliable in obesity) 2
• Do not delay paracentesis for coagulopathy correction—bleeding complications occur in only 0.2-2.2% of procedures despite platelet counts as low as 19,000 cells/mm³ and INR as high as 8.7 3
• Never order serum CA-125 in any patient with ascites—it is elevated in all patients with ascites from any cause and can lead to unnecessary surgery and death 1
• Do not assume cirrhosis is the only cause—always consider mixed ascites, as 5% have two or more simultaneous causes 3, 1
• Do not wait for culture results to treat tuberculosis—empiric therapy should be started immediately if clinical presentation strongly suggests TB 4

Prognostic Implications

Development of ascites marks significant clinical deterioration, with 5-year survival dropping from 80% to 30% in cirrhosis, and approximately 15% dying within 1 year 1. All patients developing ascites should be evaluated for liver transplantation. 1