Serum-Ascites Albumin Gradient (SAAG)
SAAG is the difference between serum albumin and ascitic fluid albumin concentrations, with a value ≥1.1 g/dL indicating portal hypertension with approximately 97% accuracy, while a value <1.1 g/dL suggests non-portal hypertensive causes of ascites. 1
Definition and Calculation
SAAG is calculated by:
- Measuring serum albumin and ascitic fluid albumin concentrations from specimens obtained on the same day
- Subtracting the ascitic fluid albumin value from the serum albumin value 2
Clinical Significance
High SAAG (≥1.1 g/dL)
Indicates portal hypertension-related ascites, which can be caused by:
- Liver cirrhosis
- Cardiac ascites
- Hepatic vein thrombosis (Budd-Chiari syndrome)
- Sinusoidal obstruction syndrome (veno-occlusive disease)
- Massive liver metastases 2, 1
Low SAAG (<1.1 g/dL)
Indicates non-portal hypertension-related ascites, which can be caused by:
- Peritoneal carcinomatosis
- Tuberculous peritonitis
- Nephrotic syndrome
- Pancreatic ascites 1
Diagnostic Utility
SAAG has replaced the traditional exudate-transudate classification system for ascites as it more accurately reflects the pathophysiologic mechanism of fluid accumulation 3. The gradient correlates with portal pressure and helps guide differential diagnosis and management strategies.
Combined Use with Total Protein
The combination of SAAG and ascitic fluid total protein levels provides additional diagnostic value:
| Parameter | Cardiac Ascites | Cirrhotic Ascites |
|---|---|---|
| SAAG | ≥1.1 g/dL | ≥1.1 g/dL |
| Total Protein | >2.5 g/dL | Generally <2.5 g/dL |
This combination helps differentiate between cardiac and cirrhotic causes of portal hypertension 1.
Clinical Pearls and Pitfalls
Repeat Paracentesis in Unexpected Results: In patients with known cirrhosis who unexpectedly have a low SAAG, a repeat paracentesis is recommended as studies show that up to 73% of such cases will demonstrate a high SAAG on repeat testing 4.
Diagnostic Yield: Evaluation of low SAAG in patients with known cirrhosis has a lower diagnostic yield compared to patients without cirrhosis 4.
Regional Variations: Some studies suggest that the optimal SAAG threshold may vary by population. For example, a study in Chinese patients proposed 12.50 g/L (1.25 g/dL) as a more appropriate cut-off value 5.
Rare Exceptions: Occasionally, conditions typically associated with high SAAG may present with low SAAG values, such as heart failure-related ascites with low SAAG 6. Similarly, malignancies like cholangiocarcinoma can rarely present with negative SAAG values 7.
Additional Testing: While SAAG is an excellent initial test, it does not differentiate between all causes of ascites. For example, tuberculous peritonitis and malignant ascites both present with low SAAG, requiring additional tests such as cytology, culture for mycobacteria, or ascitic fluid cell count 3.
SAAG remains a cornerstone in the diagnostic approach to ascites, providing valuable information about the underlying pathophysiology and guiding further diagnostic and therapeutic decisions.