What is the Serum-Ascites Albumin Gradient (SAAG) for an adult patient with suspected tuberculous peritonitis and severe hypoalbuminemia?

SAAG Calculation in Tuberculous Peritonitis with Severe Hypoalbuminemia

In a patient with suspected tuberculous peritonitis and severe hypoalbuminemia, the SAAG will typically be low (<1.1 g/dL), calculated by subtracting the ascitic fluid albumin from the serum albumin measured on the same day, though the absolute values will be reduced due to the hypoalbuminemia. 1

How to Calculate SAAG

The SAAG is calculated by measuring both serum albumin and ascitic fluid albumin on the same day, then subtracting the ascitic fluid albumin value from the serum albumin value. 1, 2

Interpretation Thresholds

• SAAG ≥1.1 g/dL: Indicates portal hypertension with approximately 97% accuracy 1, 2
• SAAG <1.1 g/dL: Indicates non-portal hypertension causes of ascites 1, 2

Expected SAAG in Tuberculous Peritonitis

Tuberculous peritonitis characteristically produces a low SAAG (<1.1 g/dL), as it represents a non-portal hypertension cause of ascites. 3, 4

Key Diagnostic Features

• In tuberculous peritonitis without cirrhosis, 100% of patients have SAAG <1.1 g/dL 4
• Ascitic fluid protein is typically >2.5 g/dL (>25 g/L) in 100% of tuberculous peritonitis cases without liver disease 4
• Ascitic fluid LDH >90 U/L occurs in 100% of tuberculous peritonitis cases without cirrhosis and 84% with coexistent liver disease 4
• Ascitic fluid lymphocytosis is typically present 1

Critical Pitfall: Tuberculous Peritonitis WITH Cirrhosis

When tuberculous peritonitis occurs in a patient with underlying cirrhosis, the SAAG may be elevated (≥1.1 g/dL) in 52% of cases due to the coexistent portal hypertension, creating diagnostic confusion. 4

Distinguishing Features in Mixed Cases

• Ascitic fluid protein may be lower (>2.5 g/dL in only 70% vs 100% without cirrhosis) 4
• However, ascitic fluid LDH >90 U/L remains highly sensitive (84%) even with coexistent cirrhosis 4
• A low LDH (<110 U/L) has 94% sensitivity and 93% specificity for tuberculous peritonitis 5
• Adenosine deaminase (ADA) >32-40 U/L has 100% sensitivity and 96.6-100% specificity in tuberculous peritonitis without cirrhosis 1

Impact of Severe Hypoalbuminemia

Severe hypoalbuminemia affects the absolute values but not the gradient interpretation—both serum and ascitic fluid albumin will be proportionally reduced, maintaining the diagnostic accuracy of the SAAG. 1, 2

Practical Considerations

• The SAAG remains valid even with severe hypoalbuminemia because it measures the gradient, not absolute values 1, 2
• In one study of cirrhotic patients with low SAAG, 73% reverted to high SAAG on repeat paracentesis, suggesting measurement variability 6
• If clinical suspicion for tuberculous peritonitis remains high despite an elevated SAAG, consider coexistent cirrhosis and proceed with ADA testing 1

Recommended Diagnostic Algorithm

For suspected tuberculous peritonitis with severe hypoalbuminemia, obtain simultaneous serum and ascitic fluid albumin, calculate SAAG, and order ascitic fluid cell count with differential, total protein, LDH, and ADA. 1, 2

Additional Testing

• Ascitic fluid ADA with cut-off >32 U/L (or >27 U/L if cirrhosis present) 1
• Ascitic fluid acid-fast bacilli smear and culture (though sensitivity is only 0-86% for smear and 20-83% for culture) 1
• If diagnosis remains uncertain, laparoscopy with peritoneal biopsy and mycobacterial culture is the gold standard 1, 4