Spontaneous bacterial peritonitis

Introduction

Spontaneous Bacterial Peritonitis (SBP) is a life-threatening infection of ascitic fluid that occurs without an identifiable intra-abdominal source. It most commonly affects patients with cirrhosis and ascites and is a medical emergency requiring urgent treatment.

Peak Incidence

• Most common in adults aged 40 to 60 years.
• Particularly prevalent in patients with decompensated liver disease.

Pathophysiology

• SBP typically occurs in patients with cirrhotic ascites due to:
  • Increased intestinal permeability.
  • Bacterial translocation from the gut.
  • Impaired hepatic and peritoneal immune responses.
• Gram-negative enteric bacteria are the most common causative organisms (e.g. E. coli, Klebsiella), though Gram-positive organisms (e.g. Streptococcus pneumoniae) may also be involved.

Symptoms

Most patients are asymptomatic or present with subtle, non-specific signs:

• Fever.
• Diffuse abdominal pain or discomfort.
• Nausea and vomiting.
• Diarrhoea or constipation.
• General malaise.

Signs

• New-onset or worsening ascites.
• Abdominal tenderness (without peritonism).
• Jaundice.
• Reduced bowel sounds or ileus.
• Altered mental status – May indicate hepatic encephalopathy or sepsis.
• Haemodynamic instability – Tachycardia, hypotension.

Diagnosis

Blood Tests

• Full Blood Count (FBC): May show leukocytosis.
• CRP / ESR: Elevated inflammatory markers.
• Liver Function Tests (LFTs): Assess for hepatic dysfunction.
• Urea & Electrolytes (U&E): To evaluate renal function.
• Clotting Screen: Essential prior to paracentesis.
• Blood Cultures: Always send two sets before starting antibiotics.

Ascitic Fluid Analysis – Gold Standard

• Diagnostic paracentesis must be performed in all patients with suspected SBP.

Key tests include:

• Cell count and differential:
  • Neutrophils ≥ 250 cells/mm³ → Diagnostic of SBP, even if culture is negative.
• Gram stain and bacterial culture.
• Appearance: Cloudy fluid may suggest infection.
• Serum–Ascites Albumin Gradient (SAAG): Assists in differentiating ascites due to portal hypertension.
• Other ascitic fluid tests:
  • Protein, glucose, LDH – May help distinguish SBP from secondary peritonitis.

Imaging

• Not required for diagnosis but may be indicated if:
  • There is new or rapidly worsening ascites.
  • Ileus is suspected.
  • There is suspicion of a secondary intra-abdominal source of infection.

Complications

• Sepsis and septic shock.
• Hepatic encephalopathy.
• Gastrointestinal bleeding.
• Acute kidney injury (AKI) or hepatorenal syndrome.
• Liver failure.
• Recurrent SBP – Common without secondary prophylaxis.

Management

Initial Management

• Empirical IV antibiotics:
  • Cefotaxime is first-line.
  • Alternatives: ceftriaxone, piperacillin–tazobactam (based on local protocols).
• IV albumin:
  • Reduces risk of renal impairment.
  • Particularly indicated if:
    • Serum bilirubin > 85 µmol/L, or
    • Serum creatinine > 88 µmol/L.

Monitoring

• Perform serial abdominal exams and monitor vital signs.
• Repeat paracentesis at 48 hours:
  • <25% reduction in neutrophil count suggests treatment failure.

Referral

• Refer for liver transplant assessment if:
  • SBP is recurrent or severe.
  • UKELD score ≥ 49 (used in the UK as a threshold for listing).

Antibiotic Prophylaxis

Consider prophylactic antibiotics in patients with ascites who are at high risk of SBP:

• Indications:

  • Previous episode of SBP.
  • Ascitic fluid protein <15 g/L plus either:
    • Child-Pugh score ≥ 9, or
    • Hepatorenal syndrome.

• Prophylactic regimens often include:

  • Norfloxacin (unlicensed in the UK but sometimes used).
  • Ciprofloxacin or co-trimoxazole (check local antimicrobial policy).