Diagnostic Threshold for Neutrophils in Spontaneous Bacterial Peritonitis
The diagnosis of SBP is confirmed when the ascitic fluid neutrophil count is >250 cells/mm³ (or >250/mm³), regardless of culture results. 1, 2
Understanding the Diagnostic Threshold
The >250 cells/mm³ cutoff has the greatest sensitivity for diagnosing SBP, making it the standard threshold used in clinical practice to avoid missing cases. 1
A higher threshold of 500 neutrophils/mm³ has greater specificity but risks missing true cases of SBP, which is clinically unacceptable given the high mortality risk. 1
The lower threshold (>250 cells/mm³) is deliberately chosen because the greater clinical risk lies with underdiagnosing SBP rather than overdiagnosing it, given that each hour of delay in treatment increases in-hospital mortality by 3.3%. 1
Alternative Diagnostic Thresholds for Context
While >250 cells/mm³ remains the diagnostic standard, understanding other thresholds helps with clinical interpretation:
PMN ≥500 cells/μL yields the highest positive likelihood ratio (10.6) and is most accurate for confirming SBP when present. 1
Total white cell count >1000 cells/μL also has high accuracy with a positive likelihood ratio of 9.1, but PMN count is the preferred metric. 1
Critical Clinical Pitfalls
Do not wait for culture results to diagnose or treat SBP—the neutrophil count alone is sufficient to initiate empirical antibiotics immediately. 1, 2, 3
Culture-negative neutrocytic ascites (PMN >250/mm³ with negative culture) should be treated identically to culture-positive SBP, as both have similar morbidity and mortality. 1, 4
Perform diagnostic paracentesis in ALL hospitalized cirrhotic patients with ascites, even without symptoms, as 16% of SBP cases are completely asymptomatic. 1, 2, 4
Bacterascites (positive culture but PMN <250/mm³) requires clinical judgment: if the patient is symptomatic, treat as SBP; if asymptomatic, repeat paracentesis as 38% will progress to frank SBP. 1