Pseudomembranous Colitis
Gastroenterology
Illness script · Gastroenterology
Pseudomembranous Colitis
C. difficile toxin-mediated colitis causing pseudomembrane formation on colonic mucosa, classically following antibiotic use.
This illness script for Pseudomembranous Colitis covers predisposing factors, classic presentation, mechanism, workup, management, and the clinical pivots that separate it from look-alikes—written for USMLE Step 1 and clerkship reasoning.
01
Predisposing factors
- Recent antibiotic use (clindamycin, fluoroquinolones, cephalosporins, ampicillin most classic)
- Hospitalized or long-term care facility patients
- Elderly (>65 years) highest risk
- Immunocompromised (HIV, chemotherapy, steroids)
- PPI use (reduces gastric acid barrier)
- Prior C. diff infection (recurrence risk ~25%)
02
Presentation
- Watery, foul-smelling diarrhea (≥3 loose stools/day); rarely bloody
- Crampy lower abdominal pain and tenderness
- Low-grade fever; leukocytosis (WBC >15,000 is a severity marker)
- Symptoms typically begin 1–10 days after antibiotic initiation (can be up to 8 weeks)
- Severe: toxic megacolon → abdominal distension, peritoneal signs, absent bowel sounds
03
Pathophysiology
- Antibiotics disrupt normal colonic flora, allowing C. difficile overgrowth
- C. diff produces toxin A (enterotoxin) and toxin B (cytotoxin) → mucosal damage
- Toxins cause epithelial cell death, neutrophil recruitment, and fibrinopurulent exudate (pseudomembranes)
- Pseudomembranes = yellow-white plaques of fibrin, mucus, necrotic debris on colonic mucosa
04
Diagnostics
- Stool C. diff toxin PCR — first-line, highly sensitive and specific
- Stool EIA (toxin A/B) — rapid but less sensitive; use in context of clinical suspicion
- Colonoscopy/sigmoidoscopy: gold standard visualization — classic yellow-white pseudomembranes
- CT abdomen: 'accordion sign' (thickened haustra) in severe disease
- Do NOT test asymptomatic patients (high false-positive rate from colonization)
05
Management
- Stop offending antibiotic if possible
- Mild-moderate: oral vancomycin 125 mg QID × 10 days OR fidaxomicin (preferred to reduce recurrence)
- Severe (WBC >15k, Cr >1.5×baseline): oral vancomycin 500 mg QID ± IV metronidazole
- Fulminant/toxic megacolon: IV metronidazole + high-dose vancomycin per rectum + surgical colectomy
- Recurrent disease: fidaxomicin, tapered vancomycin, or fecal microbiota transplant (FMT)
06
Clinical pivots
How to separate this script from the look-alikes that show up on exams and on the wards.
Infectious gastroenteritis (e.g., Salmonella, Campylobacter)
Pseudomembranous colitis requires antecedent antibiotic use; foodborne illness follows dietary exposure without antibiotic history.
Inflammatory bowel disease (ulcerative colitis)
UC is a chronic relapsing condition with bloody diarrhea; C. diff is acute post-antibiotic with toxin-positive stool.
Ischemic colitis
Ischemic colitis occurs in elderly with vascular disease and causes bloody diarrhea without prior antibiotic exposure.
Ogilvie syndrome (colonic pseudo-obstruction)
Ogilvie's causes massive colonic dilation without diarrhea or toxin; C. diff presents with profuse diarrhea and positive stool toxin.
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Educational use only. This illness script is a study framework, not medical advice. Confirm decisions with current guidelines and your clinical supervisors.