What laboratory tests are used to confirm a diagnosis of peritonitis in a patient?

Laboratory Tests for Confirming Peritonitis

The diagnosis of spontaneous bacterial peritonitis (SBP) is confirmed by an ascitic fluid polymorphonuclear (PMN) leukocyte count >250 cells/mm³, obtained via diagnostic paracentesis, regardless of culture results. 1, 2

Essential Diagnostic Tests

Primary Diagnostic Test: Ascitic Fluid Cell Count

• PMN count >250 cells/mm³ is diagnostic of SBP and should trigger immediate empirical antibiotic therapy without waiting for culture results 1
• The cell count should be performed by microscopy, as automated cell counters and reagent strips currently lack sufficient evidence for routine use 1
• Even with negative cultures, a PMN count >250 cells/mm³ confirms the diagnosis (culture-negative neutrocytic ascites occurs in approximately 50% of cases) 2, 3

Culture Studies

• Ascitic fluid culture should be inoculated into aerobic and anaerobic blood culture bottles at the bedside before starting antibiotics, which increases culture sensitivity from 50% to >90% 1, 2, 4
• Blood cultures should be obtained simultaneously before initiating antibiotics 1
• While culture is important to guide antibiotic therapy, it is not necessary for diagnosis and should never delay treatment 1

Differentiating Secondary from Spontaneous Bacterial Peritonitis

When secondary bacterial peritonitis is suspected (which has 50-80% mortality and may require surgery), additional ascitic fluid tests help differentiate it from SBP:

Key Differentiating Laboratory Tests

• Ascitic total protein ≥1 g/dL 1
• Ascitic LDH level above the normal upper limit of serum LDH 1
• Ascitic glucose concentration ≤50 mg/dL 1
• PMN count >1,000 cells/mm³ (suggests secondary peritonitis) 1
• Gram stain showing multiple organisms 1
• Ascitic CEA >5 ng/mL or alkaline phosphatase >240 U/L (suggests intestinal perforation) 1

The presence of at least two of these criteria (protein >1 g/dL, LDH > upper limit of normal, glucose <50 mg/dL) strongly suggests secondary peritonitis 2, 5

Clinical Context for Testing

When to Perform Diagnostic Paracentesis

Diagnostic paracentesis should be performed in: 1, 2

• All patients with cirrhosis and ascites at hospital admission (even without symptoms)
• Patients with gastrointestinal bleeding, shock, or hemodynamic instability
• Fever or other signs of systemic inflammation
• Abdominal pain or gastrointestinal symptoms
• Worsening liver and/or renal function
• Hepatic encephalopathy

Special Situations

• Bacterascites (positive culture but PMN <250 cells/mm³): If symptomatic, treat as SBP; if asymptomatic, repeat paracentesis as 38% will progress to frank SBP 1, 2
• **Symptomatic patients with PMN <250 cells/mm³**: Empirical antibiotics are recommended until culture results are available if signs of infection are present (fever >37.8°C, abdominal pain, or tenderness) 1

Common Pitfalls to Avoid

• Do not delay antibiotics waiting for culture results—the PMN count alone is sufficient to initiate treatment, and each hour of delay increases mortality by 3.3% 2, 3
• Do not rely on reagent strips for rapid diagnosis due to low sensitivity and high false-negative rates 1
• Do not use cytology for diagnosing infectious peritonitis—cytology is only indicated when peritoneal carcinomatosis is suspected 5
• Do not assume a single etiology—consider tuberculous peritonitis (lymphocyte predominance, adenosine deaminase >27 U/L in cirrhotics) or malignancy-related ascites when the clinical picture doesn't fit typical SBP 6, 5