What is the most likely cause of ascites in a patient with a history of tuberculosis, presenting with progressive abdominal distension, ascitic fluid analysis showing low white blood cell count and a serum-ascites albumin gradient (SAAG) suggestive of portal hypertension?

The Most Likely Cause is Malignancy (Answer D)

Based on the calculated SAAG of 2.0 g/dL (serum albumin 3.5 g/dL minus ascitic fluid albumin 1.5 g/dL), this patient has portal hypertension-related ascites, which makes liver cirrhosis (Answer B) the most likely diagnosis. 1

Critical Calculation: SAAG Analysis

The SAAG is calculated by subtracting ascitic fluid albumin (1.5 g/dL) from serum albumin (3.5 g/dL), yielding 2.0 g/dL. 1

• SAAG ≥1.1 g/dL indicates portal hypertension with 97% accuracy 1
• This patient's SAAG of 2.0 g/dL definitively places them in the high SAAG category 1

Differential Diagnosis by SAAG Category

High SAAG (≥1.1 g/dL) Causes:

• Liver cirrhosis (most common) 1
• Cardiac failure 1
• Portal vein thrombosis 1
• Budd-Chiari syndrome 1

Low SAAG (<1.1 g/dL) Causes:

• Peritoneal carcinomatosis 1
• Peritoneal tuberculosis 1
• Pancreatitis 1
• Nephrotic syndrome 1

Why Each Answer is Correct or Incorrect

Answer A (Tuberculosis) - INCORRECT

• Tuberculous peritonitis causes low SAAG ascites (<1.1 g/dL) 1
• This patient's SAAG of 2.0 g/dL excludes tuberculosis as the primary cause 1
• Tuberculous peritonitis typically shows ascitic fluid lymphocytosis, not predominantly mesothelial cells 1

Answer B (Liver Cirrhosis) - CORRECT

• Liver cirrhosis is the most common cause of high SAAG ascites, accounting for 80-85% of all ascites cases 2
• The SAAG of 2.0 g/dL confirms portal hypertension with 97% accuracy 1
• The low white cell count (80 cells/mm³) with mostly mesothelial cells indicates uncomplicated cirrhotic ascites without infection 1
• Progressive abdominal distension over weeks is consistent with decompensated cirrhosis 1

Answer C (Heart Failure) - INCORRECT

• While cardiac ascites does cause high SAAG (≥1.1 g/dL), it characteristically has high ascitic fluid protein (>2.5 g/dL) 3, 4
• This patient's ascitic fluid albumin of 1.5 g/dL suggests low total protein, more consistent with cirrhosis than cardiac failure 3, 4
• No clinical features of heart failure are mentioned 4

Answer D (Malignancy) - INCORRECT

• Peritoneal carcinomatosis causes low SAAG ascites (<1.1 g/dL) 1, 5
• This patient's high SAAG of 2.0 g/dL makes malignancy as the primary cause highly unlikely 1
• Malignant ascites would typically show malignant cells on cytology, not predominantly mesothelial cells 1

Important Clinical Pitfalls

Mixed Ascites Consideration

• Approximately 5% of patients have two or more causes of ascites ("mixed ascites") 1, 3
• When portal hypertension coexists with a second cause (e.g., peritoneal carcinomatosis or tuberculosis), the SAAG remains ≥1.1 g/dL 1, 3
• In this scenario, the portal hypertension from cirrhosis dominates the SAAG calculation 1

Low SAAG in Cirrhosis - A Rare Exception

• In rare cases, patients with known cirrhosis can have low SAAG (<1.1 g/dL), but this has low diagnostic yield 6
• When cirrhotic patients have low SAAG, 73% revert to high SAAG on repeat paracentesis 6
• Only 38% of cirrhotic patients with low SAAG have an identifiable secondary cause 6

Key Diagnostic Features Supporting Cirrhosis

• Low white cell count (80 cells/mm³) rules out spontaneous bacterial peritonitis (which requires >250 neutrophils/mm³) 1
• Predominantly mesothelial cells indicate non-inflammatory, non-infectious ascites 1
• History of tuberculosis is a red herring—completed treatment 5 years ago with wellness since makes active TB peritonitis unlikely 1