Ascites — Diagnostic Approach

SAAG-based classification & ascitic fluid workup

SAAG = Serum Albumin − Ascitic Fluid Albumin Draw both samples same day; SAAG correlates with portal pressure, not exudate/transudate

SAAG ≥ 1.1 g/dL (High) — Portal HTN

Cirrhosis (~85%)
Cardiac ascites / constrictive pericarditis
Budd-Chiari syndrome
Massive liver metastases
Portal vein thrombosis

SAAG < 1.1 g/dL (Low) — Non-portal HTN

Peritoneal carcinomatosis
Tuberculous peritonitis
Pancreatic ascites
Nephrotic syndrome
Serositis (autoimmune)

Ascitic fluid total protein further splits high-SAAG group: >2.5 g/dL suggests cardiac ascites/early Budd-Chiari; <2.5 g/dL suggests cirrhosis.

Routine + reflex fluid tests

Test	Purpose / cutoff
Cell count + differential	PMN ≥250 cells/mm³ → SBP, start empiric antibiotics
Albumin (paired w/ serum)	Calculate SAAG
Total protein	Splits high-SAAG causes; also SBP risk stratification (<1 g/dL → higher SBP risk, consider prophylaxis)
Culture (bedside inoculation into blood culture bottles)	Organism ID if SBP/secondary peritonitis suspected
Glucose, LDH	Low glucose / high LDH → suspect secondary bacterial peritonitis (bowel perforation)
ADA (adenosine deaminase)	>40 U/L suggests TB peritonitis (send if low-SAAG + lymphocytic)
Cytology	Peritoneal carcinomatosis
Amylase	Elevated in pancreatic ascites
Triglycerides	>200 mg/dL (often >1000) → chylous ascites

Secondary bacterial peritonitis clue: PMN ≥250 with ≥2 of — total protein >1 g/dL, glucose <50 mg/dL, LDH > upper limit of serum normal (Runyon criteria). Get imaging + surgical consult if suspected.

Reference: Runyon BA. AASLD Practice Guidance: Management of Adult Patients with Ascites Due to Cirrhosis 2021 (SAAG methodology, SBP diagnostic criteria). No specific Thai CPG identified for ascites diagnostic workup at time of writing.