A 52-year-old man with known cirrhosis presents with sudden onset of abdominal pain, fever (38.5°C), and clinical signs of peritonitis. Ascitic fluid analysis shows WBC 500/μL (80% neutrophils), protein 1.2 g/dL, and negative bacterial culture. Which is the most appropriate next investigation to confirm the diagnosis of spontaneous bacterial peritonitis?

Explanation

## Diagnosis of Spontaneous Bacterial Peritonitis (SBP) **Key Point:** SBP is diagnosed by ascitic fluid neutrophil count >250/μL (or PMN >250/μL) in the absence of secondary peritonitis. Culture positivity is present in only 40–60% of cases due to low bacterial inoculum. ### Why Repeat Paracentesis with Blood Culture Bottles? The **gold standard for SBP diagnosis** combines: 1. **Ascitic fluid PMN count >250/μL** (already present: 400/μL with 80% neutrophils = 320 PMN/μL) 2. **Positive bacterial culture** — obtained by inoculating ascitic fluid directly into blood culture bottles at the bedside (increases yield from ~50% to ~80%) **Clinical Pearl:** Negative culture does NOT exclude SBP if PMN count is elevated and clinical picture fits. Culture-negative neutrocytic ascites (CNNA) is treated identically to culture-positive SBP. ### Investigation Comparison Table | Investigation | Role in SBP | Sensitivity | Specificity | |---|---|---|---| | **Ascitic PMN count** | Diagnostic criterion | High | High | | **Ascitic culture (blood bottles)** | Organism identification, antibiotic targeting | 40–80% | 100% | | **Ascitic protein** | Differentiates SBP from secondary peritonitis (low protein = SBP risk) | — | — | | **CT abdomen** | Rules out secondary peritonitis (perforation, loculation) | — | — | | **Serum procalcitonin** | Marker of infection; NOT diagnostic for SBP | — | — | | **Ascitic lactate/LDH** | Non-specific markers; NOT diagnostic criteria | — | — | **High-Yield:** The diagnosis of SBP is **clinical + PMN count**. Culture is confirmatory but not required to initiate antibiotics. However, the question asks for the "most appropriate next investigation to **confirm** the diagnosis," making **culture in blood culture bottles** the single best answer. ### Management Implication Once SBP is suspected (fever + peritonitis signs + ascitic PMN >250/μL), start **cefotaxime 2 g IV 8-hourly** immediately — do NOT wait for culture results. Culture guides de-escalation if organism is susceptible to narrower agents. [cite:Harrison 21e Ch 310]