What is the diagnostic criterion for spontaneous bacterial peritonitis in ascitic fluid?

• A. Neutrophil count ≥1000 cells/µL
• B. PMN count in ascitic fluid ≥250 cells/µL
• C. Ascitic fluid total protein ≥3 g/dL
• D. Ascitic fluid culture positive for bacteria

Answer: (B)

Spontaneous bacterial peritonitis is diagnosed when the ascitic fluid shows neutrophilic inflammation, indicating infection within the peritoneal cavity. The key diagnostic threshold is a polymorphonuclear leukocyte (PMN) count in the ascitic fluid of 250 cells per microliter or higher. This cutoff is used because it best distinguishes infected from non-infected ascitic fluid, reflecting an acute neutrophilic response to bacteria.

Relying on culture results alone is not reliable, since many SBP cases have negative cultures despite infection. Conversely, a positive culture can occur without SBP if the PMN count is not elevated. Ascitic fluid total protein, while helpful for prognostic information and guiding management in cirrhosis, does not diagnose SBP. In practice, if SBP is suspected, perform paracentesis and check the PMN count; treat empirically when it meets or exceeds 250 cells/µL, even if cultures are negative.