Spontaneous Bacterial Peritonitis (SBP)

1. THE 2-MINUTE PHYSIOLOGY (Rapid Pathophysiology)

• The Core Mechanism: Spontaneous Bacterial Peritonitis (SBP) is an acute, life-threatening infection of the ascitic fluid occurring primarily in patients with liver cirrhosis, or in children with nephrotic syndrome.
• The Cellular Breakdown: The pathogenesis is driven by portal systemic hypertension causing bowel mucosal edema, which alters the gut flora and facilitates the transmural migration of enteric organisms across the bowel wall into the sterile ascitic fluid. Concurrently, hepatic dysfunction leads to impaired phagocytic function and a critical loss of opsonizing factors.
• The Culprit Organisms: The infection is classically monomicrobial, dominated by gram-negative enteric organisms such as Escherichia coli and Klebsiella. In pediatric patients with nephrotic syndrome, the loss of immunoglobulins makes them highly susceptible to encapsulated organisms like Streptococcus pneumoniae.
• The Fatal Cascade: The profound systemic inflammatory response to the infected fluid severely alters splanchnic hemodynamics, rapidly precipitating hepatorenal syndrome (acute renal failure), which develops in 30% to 50% of patients and is a massive driver of SBP mortality.

2. THE BEDSIDE ACTION PLAN (Rapid ER Management)

• Immediate Stabilization: Secure ABCs, establish IV access, and initiate rapid crystalloid resuscitation for hypoperfusion (up to 60 mL/kg in pediatric patients).
• Early Antibiotic Therapy: Do not wait for formal culture results. Following diagnostic paracentesis, immediately administer a third-generation cephalosporin, such as Cefotaxime 2 g IV every 8 hours or Ceftriaxone 2 g IV. For patients who are critically ill, have end-organ injury, recurrent SBP, or recent hospitalization, escalate to broad-spectrum agents like a carbapenem or Piperacillin-Tazobactam 4.5 g IV.
• The Albumin Resuscitation (Critical Adjunct): To prevent hepatorenal syndrome, administer IV Albumin at 1.5 g/kg within 6 hours of diagnosis, followed by 1 g/kg at 48 hours.
• Medication Cessation: Immediately discontinue all NSAIDs, ACE inhibitors, and non-selective beta-blockers.
• Symptom Control: Provide adequate analgesia; opioid administration is indicated and will not mask peritonitis. If the patient is encephalopathic, administer lactulose.

3. THE DIAGNOSTIC GRID (Differential Diagnosis & Workup)

• "Can't-Miss" Mimics:
• Secondary Bacterial Peritonitis: A polymicrobial surgical emergency caused by a perforated viscus, ruptured appendix, or abscess.
• Mesenteric Ischemia / Ischemic Bowel: Must be ruled out in older patients with vascular disease presenting with acute abdominal pain.
• Prioritized Diagnostic Workup:
• Diagnostic Paracentesis: The definitive test. Ascitic fluid must be sent for cell count with differential, Gram stain, and culture. Bedside leukocyte esterase reagent strips can be utilized as a rapid, inexpensive screening method.
• Serum-Ascites Albumin Gradient (SAAG): A SAAG > 1.1 g/dL is an early indicator of portal hypertension driving the SBP.
• Laboratory Panel: Obtain CBC, comprehensive metabolic panel (specifically looking at Creatinine and BUN), Lactate, and coagulation profile.

4. THE VISUAL BOARD (ECG / POCUS / Imaging)

• Point-of-Care Ultrasound (POCUS): Essential to confirm the presence and volume of ascites, evaluate for complex septations or loculations, and to strictly guide safe needle placement during the paracentesis to prevent iatrogenic bowel perforation.
• Fluid Inspection: Gross bedside visual inspection of the aspirated ascitic fluid may immediately reveal a turbid or cloudy appearance, raising instant suspicion for SBP before labs result.
• Abdominal Radiography/CT: Obtain abdominal imaging (plain film or CT) if you suspect secondary bacterial peritonitis to evaluate for signs of free intraperitoneal air or air-fluid levels.

5. THE SCORING MATRIX (Risk Stratification & Guidelines)

• The PMN Treatment Threshold: The absolute diagnostic criteria and treatment threshold for SBP is an ascitic fluid Polymorphonuclear (PMN/neutrophil) count > 250 cells/mm³. Note: The threshold lowers to > 100 cells/mm³ for patients currently on peritoneal dialysis.
• Albumin Administration Guidelines: IV Albumin is strictly indicated to reduce mortality if the patient exhibits any of the following parameters: Creatinine > 1.0 mg/dL (88 mmol/L), BUN > 30 mg/dL (10.7 mmol/L), or Total Bilirubin > 4 mg/dL.
• Runyon Criteria (Differentiating SBP from Secondary Peritonitis): You must suspect a surgical secondary peritonitis if $\ge$ 2 of the following are found in the ascitic fluid: Total protein > 1 g/dL, Glucose < 50 mg/dL, and LDH > the upper limit of normal for serum.

6. THE DANGER ZONE (Pitfalls & Critical Actions)

• Deadly Cognitive Trap (Continuing Home Meds): Failing to review and hold the patient's home medication list. Critical Action: You must immediately discontinue non-selective beta-blockers in patients with cirrhosis and SBP, as continuing them has been definitively shown to increase mortality.
• Deadly Cognitive Trap (Relying on Classic Signs): Expecting a rigid, surgical abdomen. Correction: SBP can be clinically quite subtle; patients may only present with unexplained clinical deterioration, low-grade fever, hypothermia, or isolated altered mental status without profound abdominal tenderness.
• Procedural Pitfall (Delaying Albumin): Focusing entirely on antibiotics and missing the renal protection. Critical Action: 30% to 50% of SBP deaths are tied to renal failure; administering weight-based IV Albumin within the first 6 hours is a mandated critical action.

7. MCQ MASTERCLASS (Written Exam Tips)

• Classic Distractor (The SBP Treatment Package): A question will describe a 45-year-old male with cirrhosis, fever, and PMNs > 250 in his ascitic fluid. The options will test your complete management plan. Distractor: "Ceftriaxone alone" or "Ciprofloxacin and metronidazole." Correction: The correct answer must be the comprehensive package: "Cefotaxime, Albumin, and discontinue beta blockade".
• Diagnostic Cutoff Fact: Board questions frequently test the absolute PMN cutoff. The answer is always an ascitic fluid neutrophil count > 250 cells/mm³.
• Pediatric Pivot: A classic pediatric EM question will present a child with profound edema, proteinuria, and sudden severe abdominal pain. Answer: This is SBP complicating Nephrotic Syndrome, commonly driven by Streptococcus pneumoniae, not appendicitis.

8. THE BOARDROOM SCRIPT (OSCE & Oral Board Tips)

• The Initial Assessment Hook: "Given this cirrhotic patient's new-onset encephalopathy, fever, and abdominal distension, I am highly concerned for Spontaneous Bacterial Peritonitis. I will perform an immediate bedside ultrasound-guided diagnostic and therapeutic paracentesis."
• The Resuscitation Command: "The ascitic fluid is grossly turbid. While waiting for the formal cell count, I will empirically order 2 grams of IV Cefotaxime. Because the fluid PMN count has returned at >250 cells/mm³ and his creatinine is elevated, I am ordering 1.5 g/kg of IV Albumin to prevent fatal hepatorenal syndrome."
• The Disposition Pivot: "I will immediately discontinue his home beta-blockers and NSAIDs to reduce his mortality risk. I am consulting Gastroenterology for hospital admission and ordering IV lactulose to manage his hepatic encephalopathy."