In spontaneous bacterial peritonitis (SBP), which diagnostic ascitic fluid finding confirms the diagnosis and mandates empirical antibiotic therapy even without a positive culture?

• A Ascites protein < 1 g/dL alone
• B Polymorphonuclear cell (PMN) count ≥ 250 cells/mm³ ✓
• C Positive ascitic fluid culture without PMN elevation
• D Ascites pH < 7.35 alone

Explanation

SBP is diagnosed when ascitic fluid PMN count ≥250 cells/mm³, regardless of culture result (culture-negative neutrocytic ascites = CNNA). Empirical therapy with cefotaxime (or ceftriaxone) should be initiated immediately on this PMN threshold since cultures are negative in up to 40% of cases. Low ascitic protein (<1.5 g/dL) is a risk factor for SBP, not diagnostic. Culture positivity alone without PMN rise suggests colonisation (bacterascites). pH is not a diagnostic criterion.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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