What is the likely diagnosis for a patient presenting with ascites and a low Serum-Ascites Albumin Gradient (SAAG)?

Low SAAG Ascites: Diagnosis

The most likely diagnosis is D. Tuberculosis (TB), as low SAAG (<1.1 g/dL) indicates ascites NOT caused by portal hypertension, thereby excluding cirrhosis, heart failure, and veno-occlusive disease—all of which produce high SAAG ascites. 1

Understanding SAAG Classification

The serum-ascites albumin gradient (SAAG) is the critical discriminator that determines the underlying pathophysiology:

• SAAG ≥1.1 g/dL (high SAAG): Indicates portal hypertension with 97% accuracy 2, 1

  • Liver cirrhosis (most common cause, accounting for 75-85% of all ascites) 2, 3
  • Heart failure 1
  • Veno-occlusive disease/Budd-Chiari syndrome 1
  • Portal vein thrombosis 1

• SAAG <1.1 g/dL (low SAAG): Indicates NON-portal hypertensive causes 2, 1

  • Peritoneal tuberculosis 1, 4
  • Peritoneal carcinomatosis 1
  • Pancreatic ascites 2
  • Nephrotic syndrome 1

Analyzing the Answer Choices

Option A (Veno-occlusive disease): INCORRECT - This causes hepatic venous outflow obstruction leading to portal hypertension and HIGH SAAG ascites 1

Option B (Liver cirrhosis): INCORRECT - Cirrhosis is the prototypical cause of HIGH SAAG ascites due to portal hypertension 2, 1

Option C (Heart failure): INCORRECT - Cardiac failure causes hepatic congestion and portal hypertension, producing HIGH SAAG ascites 1

Option D (TB): CORRECT - Tuberculous peritonitis is a classic cause of LOW SAAG ascites 1, 4

Clinical Context for TB Peritonitis

When encountering low SAAG ascites, peritoneal tuberculosis should be strongly considered, particularly in high-risk populations including:

• Patients with HIV/AIDS 5
• Recent immigrants from TB-endemic areas 5
• Immunosuppressed patients 5
• Patients with cirrhosis (paradoxically can have superimposed TB) 5

Diagnostic Features Supporting TB

• Ascitic fluid characteristics: Lymphocyte predominance with low SAAG 4, 6
• Constitutional symptoms: Fever, weight loss, progressive course over weeks to months 4, 6
• Adenosine deaminase (ADA): Levels >40 IU/L support TB diagnosis with 98% accuracy (area under ROC curve 0.98) 2, 4
• Imaging findings: Peritoneal thickening, omental involvement, loculated fluid 6

Critical Diagnostic Pitfall

Do not rely on acid-fast bacilli smear or culture alone—AFB smear sensitivity is essentially 0%, and culture positivity occurs in <50% of cases 2, 4. The diagnosis often requires laparoscopy with peritoneal biopsy, which shows granulomatous inflammation in 60-67% of cases 7. ADA levels in ascitic fluid provide a safer, more cost-effective initial diagnostic approach 6.

Other Low SAAG Causes to Consider

While TB is the answer here, the complete differential for low SAAG ascites includes:

• Peritoneal carcinomatosis: Cytology positive for malignant cells; combining with tumor markers (CEA, CA 19-9, CA 15-3) increases diagnostic yield 2
• Pancreatic ascites: Ascitic fluid amylase typically >1000 IU/L or >6 times serum amylase 2
• Nephrotic syndrome: Hypoalbuminemia with proteinuria 1