Toxin-mediated colitis after antibiotic disruption of gut microbiota; ranges from diarrhea to toxic megacolon.
Also known as: C. difficile, Clostridioides difficile, C diff, pseudomembranous colitis, CDI
Overview
Colitis caused by toxin-producing Clostridioides difficile (formerly Clostridium difficile), an anaerobic Gram-positive spore-forming bacillus. Spectrum from mild diarrhea to fulminant pseudomembranous colitis with toxic megacolon, perforation, and death.
Epidemiology
Most common healthcare-associated infection in the US; ~500,000 infections and ~30,000 deaths annually. Community-acquired CDI rising. Recurrence after first episode 15-25%; after second, 40-60%.
Try two board-style Clostridioides difficile Colitis questions
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Question 1GastrointestinalEasy
A 50-year-old man develops diarrhea with 8 stools daily, 5 days after completing a course of clindamycin for a dental abscess. He is hemodynamically stable, with a normal WBC and creatinine. Stool Clostridioides difficile toxin PCR is positive. Which of the following is the most appropriate first-line treatment?
AIntravenous metronidazole
BOral vancomycin
CFecal microbiota transplantation
DOral metronidazole
Reveal answer & full explanation
Correct answer: B — Oral vancomycin
AIntravenous metronidazole
BOral vancomycin✓
CFecal microbiota transplantation
DOral metronidazole
Why Oral vancomycin is correct
For an initial episode of Clostridioides difficile infection (CDI), current IDSA/SHEA guidance recommends oral vancomycin or oral fidaxomicin as first-line therapy for 10 days
This patient has a clear precipitant (recent clindamycin) and a positive toxin PCR with non-severe disease (hemodynamically stable, normal WBC and creatinine), so a 10-day oral vancomycin course is appropriate
Oral vancomycin is minimally absorbed and acts locally in the colonic lumen, which is why the oral route is required for CDI
Why the others are wrong
Intravenous metronidazole — reserved as an adjunct for fulminant CDI (ileus or toxic megacolon), not initial non-severe disease; this stable patient with a normal WBC does not meet that threshold (confused-with-fulminant-regimen)
Fecal microbiota transplantation — reserved for multiply recurrent CDI, not an initial episode (right-therapy-wrong-stage)
Oral metronidazole — downgraded from first-line because of inferior cure rates compared with vancomycin and fidaxomicin (premature closure on the old first-line drug)
Question 2GastrointestinalEasy
A 28-year-old male has profuse watery diarrhea (10-12 stools/day) 8 days after a week-long hospitalization for pneumonia during which he received clindamycin. Stool polymerase chain reaction (PCR) is positive for Clostridioides difficile. He has abdominal cramping but no peritoneal signs, and WBC is 18K. Fidaxomicin is unavailable on the hospital formulary. Which of the following is the most appropriate first-line treatment?
AOral vancomycin
BOral metronidazole
CIntravenous metronidazole
DFecal microbiota transplant
Reveal answer & full explanation
Correct answer: A — Oral vancomycin
AOral vancomycin✓
BOral metronidazole
CIntravenous metronidazole
DFecal microbiota transplant
Why Oral vancomycin is correct
The clinical picture (recent clindamycin, profuse watery diarrhea, positive stool PCR, no fulminant features) is a non-fulminant initial episode of Clostridioides difficile infection (CDI)
Per IDSA/SHEA 2021 guidelines, first-line therapy is oral fidaxomicin 200 mg BID or oral vancomycin 125 mg QID for 10 days; both are preferred over metronidazole
Fidaxomicin is the guideline-preferred agent (lower recurrence), but when it is unavailable, as stated in this stem, oral vancomycin is the best available first-line option
Vancomycin acts locally in the gut lumen and is not meaningfully absorbed, making it ideal for luminal CDI
Why the others are wrong
Oral metronidazole — downgraded by IDSA/SHEA 2021 to a reserve option only when neither vancomycin nor fidaxomicin is available, owing to inferior cure and higher recurrence (outdated-guideline)
Intravenous metronidazole — added to ORAL vancomycin only in fulminant CDI (hypotension, ileus, megacolon), which this patient lacks; as monotherapy it is inadequate (right-drug-wrong-setting)
Fecal microbiota transplant — reserved for multiply recurrent CDI after appropriate antibiotic courses, not a first episode (premature escalation)
Additional high-yield points
Fidaxomicin has a lower recurrence rate than vancomycin (roughly 15% vs 25%) but is costlier and not always stocked
Fulminant CDI is treated with high-dose oral vancomycin plus IV metronidazole
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Toxigenic strain BI/NAP1/027 — hypervirulent, associated with severe disease and outbreaks
Pathophysiology
Antibiotic-induced disruption of normal colonic microbiota allows C. difficile spores ingested fecal-orally to germinate. Vegetative cells produce toxins A (enterotoxin) and B (cytotoxin), and binary toxin (CDT) in some strains, which disrupt the cytoskeleton, induce inflammation, and damage colonic epithelium → pseudomembranes (fibrin, mucus, leukocytes, sloughed cells).
Clinical presentation
Symptoms
Watery diarrhea (≥3 unformed stools per 24 h) — hallmark
Lower abdominal cramping
Low-grade fever
Anorexia, nausea
Blood is unusual (consider IBD, ischemia, infection if present)
Severe disease: high fever, marked leukocytosis, severe abdominal pain, distension
Fulminant disease: shock, ileus, toxic megacolon (loss of diarrhea due to ileus)
Signs / physical exam
Abdominal tenderness
Distension (severe disease)
Fever, tachycardia
Hypotension if severe
Peritonitis suggests perforation
Diminished bowel sounds in ileus / toxic megacolon
Often normal exam in mild disease
Classic findings
Watery diarrhea, low-grade fever, leukocytosis, and abdominal pain following recent antibiotic exposure and/or hospitalization.
Differential diagnosis
Antibiotic-associated diarrhea (non-CDI) — Mild, no fever or leukocytosis, no toxin, no pseudomembranes; resolves with antibiotic cessation
CMV colitis — Immunocompromised; biopsy with inclusion bodies
Functional / IBS — Chronic, no infection, normal labs
Diagnostic workup
Diagnostic criteria
Clinical (≥3 unformed stools/24 h not otherwise explained) + positive stool test (NAAT for toxin gene, or toxin EIA, or both with multi-step algorithm). IDSA 2018 severity: Non-severe (WBC ≤15,000 AND Cr <1.5 mg/dL); Severe (WBC >15,000 OR Cr ≥1.5); Fulminant (hypotension, shock, ileus, megacolon).
Labs
Stool C. difficile testing — multi-step algorithm preferred: NAAT (PCR) alone or NAAT + toxin enzyme immunoassay
GDH antigen + toxin EIA; if discordant, reflex to NAAT
TEST ONLY in patients with clinically significant diarrhea (≥3 unformed stools/24h); do NOT test asymptomatic patients (colonization is common and not infection)
CBC — leukocytosis (WBC >15,000 = severe disease); leukemoid reaction (WBC >30,000) is poor prognostic
BMP — AKI (Cr >1.5× baseline = severe disease)
Lactate (severe disease)
Albumin (low in severe disease)
Stool studies for other pathogens if epidemiologically appropriate
Imaging
CT abdomen/pelvis — for severe or complicated disease; colonic wall thickening (mural thickening, 'accordion sign' from contrast trapped between thickened folds), pericolonic stranding, megacolon, ascites, perforation
Flexible sigmoidoscopy/colonoscopy — visualization of pseudomembranes (yellow-white plaques) is diagnostic but rarely needed and contraindicated in severe disease (perforation risk)
Diagnostic algorithm
IDSA Severity
Criteria
Preferred Therapy
Non-severe
WBC ≤15,000 AND Cr <1.5 mg/dL
Fidaxomicin 200 mg PO BID × 10 days (preferred) or oral vancomycin 125 mg QID × 10 days
Severe
WBC >15,000 OR Cr ≥1.5 mg/dL
Fidaxomicin (preferred) or oral vancomycin 125 mg QID × 10 days
Fulminant
Hypotension/shock, ileus, or megacolon
IV metronidazole 500 mg q8h + oral vancomycin 500 mg q6h (+ rectal vancomycin if ileus); early surgical consult
First recurrence
After initial cure
Fidaxomicin BID × 10 d, or tapered/pulsed vancomycin, ± bezlotoxumab
Multiply recurrent (≥2)
After multiple recurrences
Fecal microbiota transplantation (FMT) or oral microbiome therapeutics
IDSA 2018/2021 severity classification and treatment of C. difficile infection.
Treatment
First-line
DISCONTINUE inciting antibiotic if possible
Initial episode, non-severe: fidaxomicin 200 mg PO BID × 10 days (PREFERRED — lower recurrence) OR oral vancomycin 125 mg PO QID × 10 days
Initial episode, severe: fidaxomicin (preferred) or oral vancomycin 125 mg PO QID × 10 days
Fulminant disease (hypotension, shock, ileus, megacolon): IV metronidazole 500 mg q8h + oral vancomycin 500 mg q6h (and rectal vancomycin if ileus) ± early surgical consultation
Avoid antimotility agents (loperamide) in active disease — risk of toxic megacolon
Infection control: contact precautions, soap and water (alcohol does NOT kill spores), dedicated equipment
First recurrence
Fidaxomicin 200 mg BID × 10 days (preferred — lower recurrence than vancomycin)
Tapered/pulsed oral vancomycin: 125 mg QID × 10-14 d, then BID × 7 d, then daily × 7 d, then q2-3 days × 2-8 wk
Bezlotoxumab — anti-toxin B monoclonal antibody (single IV infusion adjunct to standard therapy for patients at high recurrence risk)
Multiply recurrent CDI (≥2 recurrences)
Fecal microbiota transplantation (FMT) — highly effective (~85-90% cure); via colonoscopy, enema, NG tube, capsules, or FDA-approved oral formulations (SER-109 — Vowst; RBX2660 — Rebyota)
Tapered/pulsed vancomycin or extended fidaxomicin
Add bezlotoxumab
Fulminant CDI
Combination IV metronidazole + oral high-dose vancomycin (+ rectal vancomycin if ileus)
Early surgical consultation
Surgical options: subtotal colectomy with end ileostomy (traditional), or loop ileostomy with antegrade colonic vancomycin lavage (Neal procedure — colon-sparing alternative)
Complications
Recurrence (15-25% after first episode; 40-60% after second)
Severe colitis: dehydration, electrolyte imbalance, hypotension, AKI
Toxic megacolon — colonic dilation >6 cm with systemic toxicity
Perforation
Sepsis, septic shock
Death (~5% overall; up to 80% with fulminant disease and shock)
Bowel obstruction from inflammatory stricture (rare)
Reactive arthritis (rare)
PANCE pearls
TEST ONLY symptomatic patients — colonization is common (3-15% of hospitalized adults) and testing asymptomatic patients leads to over-diagnosis.
DO NOT use IV vancomycin for CDI — does not reach colonic lumen. Oral vancomycin is essential for colonic delivery.
Fidaxomicin is now PREFERRED over vancomycin for initial CDI (IDSA 2021 update) due to lower recurrence — cost-prohibitive in some settings.
Oral metronidazole is NO LONGER first-line for initial CDI (IDSA 2017/2018) due to inferior efficacy — reserved for IV use in fulminant disease combined with oral vancomycin.
Avoid antimotility agents (loperamide, diphenoxylate) in active CDI — risk of toxic megacolon.
Alcohol-based hand sanitizer does NOT kill C. difficile spores — wash hands with soap and water.
Fecal microbiota transplantation (FMT) for recurrent CDI is highly effective (~90% cure) — FDA-approved oral formulations now available (Vowst, Rebyota).
Bezlotoxumab is a monoclonal antibody against toxin B — adjunct for patients at high risk of recurrence (age >65, prior CDI, severe disease, immunocompromised).
Toxic megacolon: colonic dilation >6 cm on imaging + systemic toxicity — STOP antimotility/opioids/anticholinergics; aggressive medical management; early surgical consultation.
Surgical options for fulminant CDI: subtotal colectomy or diverting loop ileostomy with antegrade colonic vancomycin lavage (colon-sparing).
IBD patients with CDI: continue IBD therapy; CDI worsens IBD outcomes and recurs more often.
References
IDSA/SHEA 2017 — McDonald LC et al. Clinical Practice Guidelines for Clostridium difficile Infection in Adults and Children: 2017 Update by the IDSA and SHEA. Clin Infect Dis 2018;66:e1-e48
IDSA/SHEA 2021 Focused Update — Johnson S et al. Clinical Practice Guideline by the IDSA and SHEA: 2021 Focused Update on Management of CDI in Adults. Clin Infect Dis 2021;73:e1029-e1044
ACG 2021 — Kelly CR et al. ACG Clinical Guidelines: Prevention, Diagnosis, and Treatment of Clostridioides difficile Infections. Am J Gastroenterol 2021;116:1124-1147
MODIFY I/II Bezlotoxumab — Wilcox MH et al. Bezlotoxumab for Prevention of Recurrent Clostridium difficile Infection. NEJM 2017;376:305-317
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