How to Calculate SAAG (Serum-Ascites Albumin Gradient)
The formula to calculate SAAG is: SAAG = Serum albumin - Ascitic fluid albumin, where both samples must be obtained on the same day. 1, 2
Understanding SAAG
SAAG is a crucial diagnostic tool that helps differentiate between portal hypertension-related and non-portal hypertension-related causes of ascites with approximately 97% accuracy. This calculation has been proven in prospective studies to categorize ascites better than the traditional total-protein-based exudate/transudate concept.
Interpretation of SAAG Values:
- SAAG ≥ 1.1 g/dL (11 g/L): Indicates portal hypertension (cirrhosis, heart failure, Budd-Chiari syndrome)
- SAAG < 1.1 g/dL: Suggests non-portal hypertension causes (malignancy, tuberculosis, nephrotic syndrome)
Proper Collection Technique
For accurate SAAG calculation:
- Collect serum and ascitic fluid samples on the same day
- Measure albumin concentration in both samples using the same laboratory method
- Subtract the ascitic fluid albumin value from the serum albumin value
Clinical Applications
SAAG is particularly valuable in the following scenarios:
- Initial evaluation of new-onset ascites 1
- When the cause of ascites is not immediately evident 1
- To guide treatment decisions, as high SAAG ascites typically responds to sodium restriction and diuretics 1
Common Pitfalls to Avoid
- Timing mismatch: Ensure both samples are collected on the same day
- Laboratory inconsistency: Use the same laboratory method for both measurements
- Misinterpretation: Remember that patients with mixed ascites (approximately 5% of cases) may have portal hypertension plus another cause, but will still have a SAAG ≥ 1.1 g/dL 1
- Unit conversion errors: Be consistent with units (g/dL or g/L)
Special Considerations
- In Chinese patients, some research suggests a threshold of 12.5 g/L (1.25 g/dL) may be more appropriate 3
- Patients with "mixed ascites" (two or more causes) will typically have a high SAAG if one cause is portal hypertension 1
- In rare cases, cholangiocarcinoma has been reported to produce negative SAAG values 4
SAAG has largely replaced the older exudate-transudate classification system due to its superior diagnostic accuracy (97% vs 83%) 5.