A 52-year-old man with decompensated alcoholic liver cirrhosis presents with fever, abdominal pain, and worsening ascites. Diagnostic paracentesis is performed. Which single ascitic fluid finding is MOST diagnostic of spontaneous bacterial peritonitis (SBP)?
- A Total protein < 1 g/dL
- B Positive ascitic fluid culture for a single organism
- C Polymorphonuclear (PMN) cell count ≥ 250 cells/mm³ ✓
- D Serum-ascites albumin gradient (SAAG) > 1.1 g/dL
Correct answer: C. Polymorphonuclear (PMN) cell count ≥ 250 cells/mm³
Explanation
The diagnostic criterion for SBP is an ascitic fluid PMN count ≥ 250 cells/mm³, regardless of culture results, because culture-negative neutrocytic ascites (CNNA) has the same prognosis and treatment as culture-positive SBP. A positive single-organism culture without elevated PMN count is called bacterascites and may resolve without treatment. Low total protein (< 1 g/dL) is a risk factor for SBP development, not a diagnostic criterion. SAAG > 1.1 g/dL indicates portal hypertension as the cause of ascites but is not diagnostic of SBP.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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