Ascitic Fluid Cell Count Threshold for Spontaneous Bacterial Peritonitis
An ascitic fluid polymorphonuclear (PMN) leukocyte count ≥250 cells/mm³ is diagnostic for spontaneous bacterial peritonitis and mandates immediate empirical antibiotic therapy, regardless of culture results or symptoms. 1
Diagnostic Threshold and Rationale
The 250 cells/mm³ cutoff was deliberately chosen to maximize sensitivity and prevent missed cases of SBP, prioritizing the avoidance of untreated infection over the risk of overtreatment. 1
This threshold has the highest sensitivity for SBP diagnosis, though a cutoff of 500 cells/mm³ provides greater specificity (positive likelihood ratio 10.6 vs 6.4). 1
Each hour of delay in initiating antibiotics after SBP diagnosis increases in-hospital mortality by approximately 3.3%, making the lower, more sensitive threshold clinically essential. 1
Adjustment for Bloody Ascites
- When red blood cells are present in ascitic fluid, subtract 1 PMN per 250 RBCs/mm³ to calculate the corrected PMN count. 1
Culture-Negative Neutrocytic Ascites
Approximately 40-60% of patients with PMN ≥250 cells/mm³ have negative cultures even with proper bedside inoculation into blood culture bottles. 1
These culture-negative cases carry identical morbidity, mortality, and treatment requirements as culture-positive SBP and must be treated identically. 1
Alternative Diagnostic Thresholds for Context
Total white cell count >1000 cells/μL has a positive likelihood ratio of 9.1 and high diagnostic accuracy. 1
PMN ≥500 cells/μL yields the highest positive likelihood ratio (10.6) for confirming SBP when present. 1
However, these higher thresholds should not replace the standard 250 cells/mm³ cutoff for treatment decisions, as they sacrifice sensitivity. 1
Treatment Implications Based on Cell Count
PMN ≥250 cells/mm³
Start empirical antibiotics immediately without awaiting culture results—cefotaxime 2g IV every 8-12 hours for community-acquired SBP. 1
Administer IV albumin 1.5 g/kg within 6 hours of diagnosis, then 1.0 g/kg on day 3, which reduces mortality from 29% to 10% and hepatorenal syndrome from 30% to 10%. 1
PMN <250 cells/mm³ with Symptoms
**Even when PMN <250 cells/mm³, empirical antibiotics are recommended** if the patient has fever (>37.8°C), abdominal pain/tenderness, unexplained hepatic encephalopathy, or acute kidney injury. 1
Many patients with infection symptoms but PMN <250 cells/mm³ progress to frank SBP. 1
Bacterascites (Positive Culture, PMN <250 cells/mm³)
Asymptomatic patients with bacterascites require no treatment, as approximately 62% spontaneously clear the colonization. 1
However, 38% progress to SBP—if any infection symptoms develop, treat as SBP. 1
Monitoring Treatment Response
Repeat paracentesis at 48 hours to assess treatment efficacy. 1
Treatment success is defined as PMN reduction to <25% of baseline value (i.e., ≥75% decrease) plus clinical improvement. 1, 2
Treatment failure (PMN fails to decrease by ≥25% or rises) suggests resistant organisms or secondary bacterial peritonitis—escalate antibiotics and obtain abdominal CT with surgical consultation. 1
In one study, all patients with PMN ≥250 cells/mm³ at 48 hours experienced SBP recurrence, while only 1 patient with PMN <250 cells/mm³ at 48 hours had recurrence. 2
Differentiating Secondary Bacterial Peritonitis
Secondary peritonitis requires surgical intervention and has 50-80% mortality if untreated. 1 Suspect secondary peritonitis when:
- PMN count >1000 cells/mm³ 1
- Multiple organisms on Gram stain or culture 1
- Ascitic total protein ≥1 g/dL 1
- Ascitic LDH above normal serum upper limit 1
- Ascitic glucose ≤50 mg/dL 1
- PMN count fails to decrease after 48 hours of appropriate antibiotics 1
- Ascitic CEA >5 ng/mL or alkaline phosphatase >240 U/L (suggests intestinal perforation) 1
Critical Pitfalls to Avoid
Never delay paracentesis in hospitalized cirrhotic patients, even if asymptomatic—up to one-third of SBP cases present without symptoms. 1
Never wait for culture results before starting antibiotics when PMN ≥250 cells/mm³—the cell count alone justifies immediate therapy. 1
Do not use reagent strip tests for SBP diagnosis due to low sensitivity and high false-negative rates. 1
The 250 cells/mm³ threshold is specific for bacterial peritonitis—do not apply this cutoff to tuberculous peritonitis, which presents with lymphocyte predominance rather than neutrophil predominance. 3
Inoculate ≥10 mL ascitic fluid into blood culture bottles at bedside before antibiotics to achieve >90% culture sensitivity (vs ~50% with conventional methods). 1