Guideline Summary: Classification and Diagnostic Approach to Ascitic Fluid

Primary Classification – Serum‑Ascites Albumin Gradient (SAAG)

The SAAG, calculated as serum albumin – ascitic fluid albumin obtained on the same day, is the preferred initial test because it distinguishes portal‑hypertensive from non‑portal‑hypertensive ascites with 97 % accuracy【1】.
SAAG ≥ 1.1 g/dL indicates portal hypertension (e.g., cirrhosis, cardiac ascites, Budd‑Chiari syndrome, sinusoidal obstruction syndrome) with 97 % accuracy【1】.
SAAG < 1.1 g/dL indicates non‑portal‑hypertensive causes (e.g., peritoneal carcinomatosis, tuberculous peritonitis, pancreatic ascites)【1】.

A polymorphonuclear (PMN) cell count ≥ 250 cells/mm³ in ascitic fluid defines SBP and mandates immediate empiric antibiotics【4】.
Even when PMN < 250 cells/mm³, the presence of clinical infection signs (fever, abdominal pain/tenderness) warrants empiric therapy【4】.

High SAAG (≥ 1.1 g/dL) + ascitic protein > 2.5 g/dL specifically points to cardiac ascites【2】.
In contrast, cirrhotic ascites typically shows high SAAG with protein < 2.5 g/dL【2】.

The presence of any two of the following suggests gut perforation: ascitic total protein > 1 g/dL, ascitic LDH above the serum upper‑limit of normal, and ascitic glucose < 50 mg/dL【3】.
Multiple organisms on Gram stain or culture further support secondary peritonitis【4】.
CEA > 5 ng/mL or alkaline phosphatase > 240 U/L provides 92 % sensitivity and 88 % specificity for gut perforation【4】.

Ascitic fluid lymphocytosis is a typical cytologic pattern【1】.
Adenosine deaminase (ADA) > 32–40 U/L yields 100 % sensitivity and 96.6–100 % specificity in non‑cirrhotic patients【1】.
Acid‑fast bacilli (AFB) smear (0–86 % sensitivity) and culture (20–83 % sensitivity) should be performed to confirm infection【1】.

Obtain cell count with differential, ascitic albumin (for SAAG), total protein, and inoculate bedside blood‑culture bottles for bacterial culture【1】【5】.

Approximately 5 % of patients have “mixed ascites” with multiple etiologies, requiring a comprehensive work‑up【2】.
Delayed specimen transport compromises cell‑count accuracy and culture yield; samples must be sent to the laboratory promptly【5】.