How to Calculate SAAG
To calculate the serum-ascites albumin gradient (SAAG), measure the serum albumin concentration and ascitic fluid albumin concentration on the same day, then subtract the ascitic fluid albumin value from the serum albumin value 1, 2.
The Calculation Formula
SAAG = Serum Albumin (g/dL) - Ascitic Fluid Albumin (g/dL)
Critical Technical Requirements
- Both specimens must be obtained on the same day to ensure accuracy 1, 2
- Measure albumin concentrations in both serum and ascitic fluid using standard laboratory methods
- No correction factors or adjustments are needed for the basic calculation
Interpretation of Results
High SAAG (≥1.1 g/dL or ≥11 g/L)
- Indicates portal hypertension with approximately 97% accuracy 1, 2
- Common causes include:
- Cirrhosis (most common)
- Cardiac ascites
- Hepatic vein thrombosis (Budd-Chiari syndrome)
- Sinusoidal obstruction syndrome
- Portal vein thrombosis
Low SAAG (<1.1 g/dL or <11 g/L)
- Excludes portal hypertension as the primary cause 3
- Common causes include:
- Peritoneal carcinomatosis
- Tuberculous peritonitis
- Nephrotic syndrome
- Pancreatic ascites
Clinical Context and Pitfalls
Important caveat: Patients with portal hypertension plus a second cause of ascites (mixed ascites, occurring in approximately 5% of cases) will still have a SAAG ≥1.1 g/dL 1. The SAAG reflects the presence of portal hypertension regardless of additional contributing factors.
The SAAG has replaced the outdated exudate/transudate classification system for ascites, as it has been proven superior in prospective studies for categorizing ascites etiology 1, 4.
When to Order SAAG
According to current guidelines 5:
- Always order for new-onset ascites (both inpatients and outpatients)
- Not needed for recurrent ascites in patients with established diagnosis
- Should be part of initial ascitic fluid analysis along with cell count/differential and total protein
Treatment Implications
The SAAG result directly guides management 1, 6:
- High SAAG ascites: Typically responds to sodium restriction and diuretics
- Low SAAG ascites: Usually does not respond to salt restriction and diuretics (except nephrotic syndrome), requiring treatment of the underlying disorder