What is the normal range of protein in ascitic fluid?

Normal Range of Protein in Ascitic Fluid

The normal range of protein in ascitic fluid varies based on etiology, with cirrhotic ascites typically having a total protein concentration <1.5 g/dL, while cardiac ascites typically has >2.5 g/dL. 1, 2

Protein Levels by Etiology

Ascitic fluid protein levels vary significantly depending on the underlying cause:

• Cirrhotic ascites:

  • Total protein typically <1.5 g/dL 1
  • Low protein (<1.5 g/dL) is considered a risk factor for spontaneous bacterial peritonitis (SBP) 1

• Cardiac ascites:

  • Total protein typically ≥2.5 g/dL 3
  • Higher protein levels despite having high SAAG (≥1.1 g/dL) 2

• Malignancy-related ascites:

  • Generally higher protein levels than cirrhotic ascites 4
  • Often >2.5 g/dL

• Tuberculous peritonitis:

  • Higher protein levels than cirrhotic ascites 4
  • Often >2.5 g/dL

Clinical Significance of Ascitic Fluid Protein

Risk for Spontaneous Bacterial Peritonitis (SBP)

• Total protein concentration <1.5 g/dL has traditionally been considered a risk factor for SBP in cirrhotic patients 1
• However, recent evidence suggests that low ascitic fluid protein alone may not be a reliable predictor of SBP development 5
• Complement 3, an important component of ascitic fluid protein, offers local defense against infection 6

Differential Diagnosis

• SAAG (Serum-Ascites Albumin Gradient) is more useful than total protein for differentiating causes of ascites:

  • SAAG ≥1.1 g/dL indicates portal hypertension with 97% accuracy 2
  • SAAG <1.1 g/dL suggests non-portal hypertensive causes 2

• Combined parameters provide better diagnostic accuracy:

  • High SAAG + low protein (<2.5 g/dL) = cirrhotic ascites
  • High SAAG + high protein (≥2.5 g/dL) = cardiac ascites 2, 3
  • Low SAAG + high protein = malignancy or tuberculosis 4

Secondary Bacterial Peritonitis vs. SBP

When evaluating for secondary bacterial peritonitis (requiring surgical intervention), protein levels can help:

• Total protein >1 g/dL (along with LDH greater than upper limit of normal for serum and glucose <50 mg/dL) suggests secondary bacterial peritonitis from visceral perforation 1

Practical Approach to Ascitic Fluid Analysis

1. Always calculate SAAG = serum albumin - ascitic albumin

   • SAAG ≥1.1 g/dL indicates portal hypertension
   • SAAG <1.1 g/dL suggests non-portal hypertensive causes

2. Measure total protein concentration

   • <1.5 g/dL: typical of cirrhotic ascites, traditionally considered risk factor for SBP

   • 2.5 g/dL: suggests cardiac ascites (if SAAG high) or non-portal hypertensive causes (if SAAG low)

3. Additional tests when infection suspected

   • Cell count with differential (PMN >250/mm³ indicates SBP)
   • Culture (bedside inoculation into blood culture bottles)
   • Glucose, LDH (to rule out secondary bacterial peritonitis)

Common Pitfalls

• Relying solely on protein levels for diagnosis without calculating SAAG
• Assuming all patients with low protein (<1.5 g/dL) need SBP prophylaxis without considering other risk factors
• Failing to consider mixed ascites (approximately 5% of cases) where multiple etiologies coexist 2
• Not obtaining serum albumin on the same day as paracentesis, which can lead to inaccurate SAAG calculation