On paracentesis, which finding confirms spontaneous bacterial peritonitis?

• A. Positive ascitic fluid culture
• B. Elevated ascitic protein
• C. PMN count > 250 cells/mcL
• D. Elevated LDH

Answer: (C)

The key idea is that spontaneous bacterial peritonitis is diagnosed by the neutrophil response in the ascitic fluid, not by culture results or other nonspecific markers. A polymorphonuclear leukocyte count higher than 250 cells per microliter in ascitic fluid confirms SBP because it reflects an acute neutrophilic infection within the peritoneal cavity. Cultures may be positive in some cases, but many patients with SBP have negative cultures; therefore, relying on the PMN threshold captures both culture-positive and culture-negative infections. Elevated ascitic protein or LDH can occur for various reasons and do not specifically indicate SBP, so they aren’t diagnostic.