What is the polymorphonuclear leukocyte (PMN) count criteria for diagnosing spontaneous bacterial peritonitis (SBP)?

Diagnostic Criteria for Spontaneous Bacterial Peritonitis

The diagnosis of spontaneous bacterial peritonitis (SBP) is confirmed when the ascitic fluid polymorphonuclear leukocyte (PMN) count is ≥250 cells/mm³, regardless of culture results. 1

Core Diagnostic Threshold

• An ascitic fluid PMN count ≥250 cells/mm³ establishes the diagnosis of SBP and mandates immediate empirical antibiotic therapy, even before culture results are available 1, 2
• This threshold of 250 cells/mm³ provides the greatest sensitivity for detecting SBP, though a higher threshold of 500 cells/mm³ offers greater specificity 1
• The diagnosis is made in the absence of an evident intra-abdominal, surgically treatable source of infection 1

Important Adjustments and Considerations

Hemorrhagic Ascites Correction

• If red blood cells are present in the ascitic fluid, subtract 1 PMN per 250 RBCs/mm³ to obtain the corrected PMN count 1

Culture-Negative Neutrocytic Ascites

• Approximately 40% of patients with PMN count ≥250/mm³ will have negative cultures even with appropriate bedside inoculation into blood culture bottles 1
• These culture-negative cases should be treated identically to culture-positive SBP, as they demonstrate similar clinical courses and mortality 1

Symptomatic Patients with Lower PMN Counts

• **Even if the PMN count is <250/mm³, empirical antibiotics should be initiated if signs or symptoms of infection are present** (fever >37.8°C, abdominal pain or tenderness, unexplained renal impairment, or hepatic encephalopathy) 1
• Studies show that symptomatic patients with PMN counts <250/mm³ can progress to frank SBP 1

Alternative Diagnostic Thresholds for Clinical Context

The 2021 Gut guidelines provide nuanced likelihood ratios for different thresholds 1:

• PMN ≥500 cells/μL yields the highest positive likelihood ratio of 10.6 (95% CI 6.1-18.3)
• PMN >250 cells/μL has a positive likelihood ratio of 6.4 (95% CI 4.6-8.8)
• The lower threshold of 250 cells/mm³ remains standard in routine practice because the greater clinical risk lies in underdiagnosing SBP 1

Distinguishing Secondary Bacterial Peritonitis

PMN count >1,000/mm³ should raise suspicion for secondary bacterial peritonitis rather than spontaneous peritonitis 1:

• Secondary peritonitis typically shows PMN counts in the thousands
• Additional features include multiple organisms on Gram stain, ascitic total protein ≥1 g/dL, LDH above normal serum limits, and glucose ≤50 mg/dL 1
• These patients require imaging (CT scan) and potential surgical intervention 1

Practical Diagnostic Approach

When to Perform Diagnostic Paracentesis

• All patients with cirrhosis and ascites admitted to hospital, even without symptoms 2
• Any patient with gastrointestinal bleeding, shock, fever, abdominal pain, worsening liver/renal function, or hepatic encephalopathy 2

Laboratory Processing

• Inoculate ascitic fluid into blood culture bottles at bedside before antibiotics are given, as this increases culture yield from approximately 50% to 80% 1
• Automated blood cell counters are reliable for rapid PMN determination with 94% sensitivity and 100% specificity 3
• A total ascitic nucleated cell count <1.0 g/L has a 95.5% negative predictive value for excluding SBP 4

Common Pitfalls to Avoid

• Do not delay antibiotics waiting for culture results if PMN count is ≥250/mm³ 1, 2
• Do not rely on reagent strip tests (leukocyte esterase) as they have low sensitivity and high false-negative rates 1
• Do not make a "clinical diagnosis" without paracentesis—ascitic fluid analysis is mandatory for confident diagnosis 1
• Even a single dose of antibiotics before paracentesis causes cultures to be negative in 86% of cases 1