Ascitic fluid: SAAG and infection

What it is

Ascites is fluid in the peritoneal cavity, and in most adults it is caused by portal hypertension from cirrhosis. A diagnostic paracentesis answers two questions. First, why is the fluid there? The serum-ascites albumin gradient (SAAG) subtracts the ascitic albumin from a serum albumin drawn the same day. A gradient of 1.1 g/dL or more means the fluid is driven by high pressure in the portal system (cirrhosis, alcoholic hepatitis, heart failure, Budd–Chiari syndrome, portal vein thrombosis); a gradient under 1.1 g/dL points to peritoneal disease or low albumin without portal hypertension (cancer of the peritoneum, tuberculosis, pancreatitis, nephrotic syndrome). The SAAG has largely replaced the old transudate–exudate idea for ascites. The ascitic total protein refines it: high protein with a high SAAG suggests the liver sinusoids are intact (cardiac ascites, Budd–Chiari), and low protein in cirrhosis means weak defences against infection. Second, is it infected? The neutrophil (PMN) count decides: 250 cells/mm³ or more is spontaneous bacterial peritonitis (SBP) unless there is a surgical source, and treatment starts on that number without waiting for culture.

Reference values and concepts

• Basic tests on every diagnostic tap: cell count with differential, albumin, total protein, and culture inoculated into blood culture bottles at the bedside.
• Bedside inoculation into blood culture bottles raises the culture yield compared with sending fluid in a plain tube.
• Ascitic total protein under about 1.5 g/dL in cirrhosis marks a higher risk of SBP.
• In bloody fluid, subtract about one neutrophil for every 250 red cells before reading the PMN count.
• Optional tests by suspicion: glucose, LDH, amylase (pancreatic), triglycerides (chylous, usually over 200 mg/dL), cytology, mycobacterial tests and ADA.
• SBP is usually monomicrobial, most often Gram-negative bowel bacteria such as Escherichia coli and Klebsiella.

Reading it step by step

1. Look at the fluid — straw is typical of cirrhosis; cloudy suggests infection; milky suggests chyle; bloody suggests trauma or cancer.
2. Calculate the SAAG with same-day serum albumin — this sorts the cause into portal hypertension or not.
3. SAAG of 1.1 g/dL or more: read the total protein — under 2.5 g/dL fits cirrhosis; 2.5 g/dL or more fits heart failure or Budd–Chiari.
4. SAAG under 1.1 g/dL: think peritoneum and protein loss — cancer, tuberculosis, pancreatitis or nephrotic syndrome.
5. Read the neutrophil count, corrected for red cells — 250 cells/mm³ or more is SBP and treatment starts now.
6. Check for secondary peritonitis — protein over 1 g/dL, glucose under 50 mg/dL and LDH above serum normal suggest perforation or abscess.
7. Read culture, cytology and special tests when they return — they confirm the organism or reveal cancer or tuberculosis.

Ascitic fluid patterns and likely causes (ranges vary by laboratory).

Patterns and common mistakes

• Not tapping a patient with cirrhosis admitted for another reason: SBP is often silent or shows only as encephalopathy or kidney injury.
• Waiting for a positive culture before treating: culture is negative in many true SBP cases.
• Using an albumin from a different day for the SAAG, which makes the gradient unreliable.
• Calling a polymicrobial infection SBP and missing a bowel perforation that needs surgery.
• Forgetting the red cell correction in bloody fluid and overcalling infection.
• Reading a high SAAG as «cirrhosis» without thinking of heart failure, Budd–Chiari or portal vein thrombosis.

Next step

• SBP: start an empirical third-generation cephalosporin and intravenous albumin to protect the kidneys; avoid nephrotoxic drugs.
• Suspected secondary peritonitis: urgent abdominal CT and surgical review; broaden antibiotic cover.
• Call a senior for SBP with shock, rising creatinine, gastrointestinal bleeding or encephalopathy.
• Repeat the tap after about 48 hours of treatment if the patient does not improve or the organism is unusual, to confirm the neutrophils fall.
• After a first SBP, discuss long-term antibiotic prophylaxis and referral for liver transplant assessment.
• Low SAAG with cytology or tuberculosis tests positive: refer to oncology or start the tuberculosis pathway.

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Elsewhere in DurgiPedia

• Ascites: shifting dullness and fluid wave
• Portal hypertension and the signs of chronic liver disease
• Serum albumin as a lab test
• Starling forces at the capillary
• Portal hypertension

Connections

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Ascites (StatPearls (NCBI Bookshelf), 2025); Paracentesis (StatPearls (NCBI Bookshelf), 2025).

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