Gastrointestinal

C. difficile infection: diagnosis and treatment by severity

Clinical Overview and When to Suspect C. difficile Infection

— Healthcare-associated (HA-CDI): hospitalized or recently discharged patients, long-term care residents

— Community-associated (CA-CDI): increasingly common, often younger patients without classic risk factors

— Recurrent CDI: 20–25% after a first episode, 40–60% after a second

— Recent antibiotic exposure within 8–12 weeks (clindamycin, fluoroquinolones, cephalosporins, carbapenems, broad-spectrum penicillins are highest risk)

— Recent hospitalization or healthcare exposure

— Age ≥65

— PPI use, immunosuppression, IBD, chemotherapy

— Gastric surgery, tube feeds, recent GI surgery

— Ileus or toxic megacolon with paradoxically little or no diarrhea — high mortality, often missed

— Leukemoid reaction (WBC >30k) with vague abdominal pain in a post-op or post-antibiotic patient

— Postoperative fever + diarrhea in a patient who received perioperative cefazolin

Board pearl: A positive C. difficile NAAT/PCR in a patient with formed stool or only 1–2 loose stools likely reflects colonization, not infection — treat the patient, not the test. Step 3 stems often bait you with a positive PCR in a patient whose diarrhea is more consistent with laxatives, tube feeds, or recent contrast administration.

C. difficile infection (CDI) is a toxin-mediated colitis caused by Clostridioides difficile, a Gram-positive spore-forming anaerobe; toxins A (TcdA) and B (TcdB) disrupt colonocyte cytoskeleton → inflammation, pseudomembrane formation, secretory diarrhea.

Epidemiology pattern Step 3 loves:

When to suspect — the trigger phrase is "≥3 unformed stools in 24 hours" plus a risk factor:

Atypical presentations to recognize:

Do not test asymptomatic patients or those on laxatives — colonization is common (up to 50% of inpatients) and a positive PCR in a colonized patient leads to overtreatment.

Presentation Patterns and Key History

— Mild–moderate: 3–10 loose stools/day, mild crampy lower abdominal pain, low-grade fever, WBC <15,000

— Severe: WBC ≥15,000 or creatinine >1.5 mg/dL (IDSA 2021)

— Fulminant (previously "severe-complicated"): hypotension/shock, ileus, or toxic megacolon

— Antibiotic timeline: what, when, how long — include single perioperative doses and recent dental antibiotics

— Hospitalizations, ED visits, dialysis center exposure in past 12 weeks

— Nursing home or rehab residence

— Chemotherapy, biologics (especially anti-TNF), corticosteroids

— PPI/H2 blocker use — often missed OTC

— Prior CDI episodes and what was used to treat them (vancomycin vs fidaxomicin vs metronidazole)

— IBD history — CDI both mimics and triggers flares

— Sudden cessation of diarrhea with worsening pain/distension → suspect ileus or megacolon

— Lightheadedness, decreased urine output → volume depletion/sepsis

— Bloody stools are uncommon in pure CDI; if present, consider concurrent ischemic colitis or IBD flare

Step 3 management: In the ambulatory setting, a patient calling 5 days after a dental course of clindamycin with new watery diarrhea should be brought in for stool testing and exam — do not empirically treat over the phone, and do not prescribe loperamide, which can precipitate toxic megacolon. Stop the inciting antibiotic if still ongoing and clinically possible.

Cardinal symptom: new-onset watery, non-bloody diarrhea, often described as having a characteristic foul odor (do not rely on smell alone — low specificity).

Severity spectrum on the wards:

Key history elements to extract on the H&P:

Red-flag history points suggesting fulminant disease:

Occupational/social: healthcare workers, daycare exposure, recent travel (rule out other causes but does not exclude CDI).

Physical Exam Findings and Hemodynamic Assessment

— Tachycardia >100, hypotension (SBP <90 or MAP <65), or need for vasopressors = fulminant CDI

— Fever >38.5°C, especially with leukocytosis, raises concern for severe disease

— Orthostatics in outpatients to gauge volume status

— Mild–moderate: diffuse or lower abdominal tenderness, hyperactive bowel sounds

— Severe: more pronounced tenderness, often with guarding

— Fulminant: distension, diminished or absent bowel sounds (ileus), rebound/rigidity (peritonitis from perforation or megacolon)

— Tympany to percussion + tenderness in a CDI patient = obtain urgent imaging for megacolon

— Altered mental status, cool extremities, mottling, decreased capillary refill

— Oliguria

— Lactic acidosis on labs (covered next chunk)

— Dry mucous membranes, decreased skin turgor, flat JVP

— Capillary refill >3 seconds

— Calculate fluid deficit; isotonic crystalloid (LR preferred over NS in large volumes to avoid hyperchloremic acidosis)

CCS pearl: On the CCS case, for any suspected fulminant CDI, the order set should include: continuous pulse oximetry, telemetry, two large-bore IVs, IV fluids (LR), NPO, NG tube if ileus, serial abdominal exams q4h, surgical consult, and abdominal imaging — all within the first simulated hour. Do not wait for stool PCR to act on hemodynamic instability; empiric oral vancomycin + IV metronidazole should be started.

General appearance: ranges from well-appearing outpatient with crampy pain to toxic, obtunded ICU patient with distended abdomen.

Vitals — the severity discriminator:

Abdominal exam:

Systemic signs of sepsis/end-organ hypoperfusion:

Volume assessment is critical and frequently tested:

DRE: generally unrevealing in CDI; do not perform if megacolon suspected (risk of perforation).

Diagnostic Workup — Initial Labs and Imaging

— Two-step algorithm (preferred when no preagreed clinical criteria):

— Step 1: GDH (glutamate dehydrogenase) antigen + toxin A/B EIA

— If discordant (GDH+/toxin−): reflex to NAAT/PCR

— NAAT alone acceptable only if institution restricts testing to symptomatic patients with ≥3 unformed stools/24h and no laxatives in 48h

— Toxin EIA alone has poor sensitivity — not recommended as standalone

— NAAT detects the toxin gene, not active toxin → positive in colonization

— Toxin EIA confirms active toxin production → better specificity but lower sensitivity

— Test asymptomatic patients (including "test of cure" after treatment)

— Repeat testing within 7 days of an initial result

— Test patients on laxatives or tube feeds without clinical reassessment

— CBC: WBC ≥15,000 = severe; WBC >25,000 or bandemia → concern for fulminant

— BMP: creatinine >1.5 mg/dL = severe; track for AKI

— Lactate ≥5 mmol/L in fulminant CDI predicts mortality; >2.2 warrants urgent surgical eval

— Albumin <3 g/dL correlates with severity

— Type and screen if surgery possible

— Abdominal radiograph: screen for ileus, megacolon (transverse colon diameter >6 cm), free air

— CT abdomen/pelvis with contrast: colonic wall thickening, "accordion sign," pericolonic stranding, ascites; mandatory if fulminant disease or unclear diagnosis

Board pearl: A NAAT-positive patient with a negative toxin EIA who is clinically improving without therapy likely has colonization — Step 3 expects you to reassess clinical criteria rather than reflexively treating.

Stool testing — only on unformed/loose stool from a symptomatic patient. Two acceptable algorithms per IDSA/SHEA 2021:

Why this matters on Step 3:

Do NOT:

Supporting labs:

Imaging:

Diagnostic Workup — Advanced and Confirmatory Studies

— Not routinely needed for diagnosis

— Reserve for: diagnostic uncertainty, ileus without diarrhea where stool testing impossible, suspected alternative diagnosis (IBD flare, ischemic colitis)

— Finding: yellow-white pseudomembranes on erythematous, friable mucosa is essentially pathognomonic, though absent in 20–30% of confirmed cases

— Risks: perforation, especially if megacolon — insufflate minimally, stop early if findings clear

— Marked colonic wall thickening (>4 mm), often pancolonic

— Accordion sign: alternating high- and low-attenuation bands from contrast trapped between thickened haustra

— Target sign: alternating mucosal/submucosal layers

— Pericolonic stranding, ascites

— Free air → perforation → emergent surgery

— Radiographic colonic dilation >6 cm

— Plus ≥3 of: fever >38°C, HR >120, WBC >10,500, anemia

— Plus ≥1 of: dehydration, AMS, electrolyte disturbance, hypotension

— Stool culture for Salmonella, Shigella, Campylobacter, Yersinia if travel/food exposure or atypical features

— Shiga toxin EIA if bloody diarrhea

— Giardia/Cryptosporidium antigen in immunocompromised or persistent diarrhea

— Norovirus PCR in outbreak settings

Key distinction: Pseudomembranes seen on endoscopy are highly specific for CDI but not required for diagnosis — diagnosis is clinical + microbiologic. Conversely, absence of pseudomembranes does not exclude CDI, particularly in IBD or partially treated patients.

Flexible sigmoidoscopy or colonoscopy:

CT findings in detail:

Toxic megacolon criteria (Jalan):

Concurrent workup to exclude alternatives:

In IBD patients: test for CDI at every flare — coinfection changes management drastically (often hold biologic escalation until CDI controlled).

Biomarkers under study (not yet standard): fecal lactoferrin, calprotectin — helpful in IBD overlap but do not substitute for toxin assays.

Severity Stratification and First-Line Management Logic

— Non-severe (mild–moderate): WBC <15,000 AND Cr <1.5 mg/dL

— Severe: WBC ≥15,000 OR Cr ≥1.5 mg/dL

— Fulminant: hypotension/shock, ileus, OR megacolon (regardless of labs)

— Initial non-severe or severe episode:

— Preferred: Fidaxomicin 200 mg PO BID × 10 days

— Alternative: Vancomycin 125 mg PO QID × 10 days

— Metronidazole only if neither available and disease is non-severe

— Fulminant:

— Vancomycin 500 mg PO/NG QID PLUS metronidazole 500 mg IV q8h

— Add vancomycin per rectum 500 mg in 100 mL saline q6h if ileus present

— Urgent surgical consult

— Stop the inciting antibiotic if possible; if not, narrow spectrum

— Discontinue PPI if not strongly indicated

— Avoid antimotility agents (loperamide, opioids) in moderate-severe disease

— Aggressive IV fluid resuscitation, electrolyte correction

— Contact precautions with soap and water handwashing (alcohol gel does not kill spores)

— Private room; dedicated equipment

— Outpatient: hemodynamically stable, tolerating PO, reliable, mild–moderate disease, no significant comorbidity

— Admit: severe disease, fulminant features, unable to tolerate PO, hemodynamic instability, significant comorbidity, social factors limiting outpatient management

Step 3 management: Fidaxomicin is now preferred over vancomycin for both initial and first recurrent CDI due to lower recurrence rates (sustained clinical response advantage ~10%). Cost is the practical barrier; if insurance denies, vancomycin PO is fully acceptable first-line on the exam.

IDSA/SHEA 2021 severity classification (memorize):

First-line therapy by category (2021 update — fidaxomicin promoted):

General management principles:

Outpatient vs inpatient decision:

Pharmacotherapy — Detailed Regimens

— Macrocyclic antibiotic, minimal systemic absorption, narrow spectrum (preserves microbiome)

— Lower recurrence vs vancomycin

— Alternative: extended-pulsed regimen (200 mg BID days 1–5, then 200 mg every other day days 7–25) — useful for recurrence prevention in high-risk patients

— Not systemically absorbed (do not check trough levels)

— IV vancomycin has no role in CDI (does not reach colon lumen)

— For fulminant: increase to 500 mg PO/NG QID ± PR

— Vancomycin tapered/pulsed regimen for recurrence: 125 mg QID × 10–14d → BID × 7d → daily × 7d → every 2–3 days × 2–8 weeks

— IV 500 mg q8h only as adjunct in fulminant disease or when oral route unavailable

— PO use limited to settings where vancomycin/fidaxomicin unavailable and disease is non-severe

— Adverse effects: peripheral neuropathy with prolonged use (>14 days), disulfiram reaction with alcohol, metallic taste

— Monoclonal antibody against toxin B

— Single IV infusion (10 mg/kg) given with standard antibiotic therapy

— Indicated as adjunct to reduce recurrence in high-risk patients: age ≥65, immunocompromised, severe CDI, prior CDI episode in past 6 months

— Boxed warning: heart failure exacerbation — caution in CHF

— Probiotics: not routinely recommended by IDSA (insufficient evidence, risk of bacteremia in immunocompromised)

— Cholestyramine: binds vancomycin — do not co-administer

— IV immunoglobulin: no proven benefit, not recommended

Board pearl: IV vancomycin does not treat CDI — it doesn't reach the colonic lumen. A common Step 3 distractor is "switch to IV vancomycin" in a patient with worsening CDI; the correct answer is to escalate oral/rectal vancomycin and add IV metronidazole, plus surgical consult.

Fidaxomicin 200 mg PO BID × 10 days:

Vancomycin PO 125 mg QID × 10 days:

Metronidazole:

Bezlotoxumab:

Adjunctive considerations:

Recurrent CDI and Procedural/Microbiome-Based Therapies

— Fidaxomicin 200 mg BID × 10 days (preferred), especially if vancomycin used initially

— Fidaxomicin extended-pulsed regimen

— Vancomycin tapered/pulsed if fidaxomicin was used initially

— Add bezlotoxumab as adjunct if high-risk

— Fidaxomicin (standard or extended-pulsed)

— Vancomycin taper/pulse

— Vancomycin × 10 days followed by rifaximin 400 mg TID × 20 days ("chaser")

— Fecal microbiota transplantation (FMT) — strongly recommended after ≥2 recurrences (i.e., 3rd episode)

— Cure rates 80–90% for recurrent CDI

— Routes: colonoscopy (highest efficacy), enema, NG/ND tube, oral capsules

— FDA-approved products: Rebyota (fecal microbiota suspension, rectal) and Vowst (oral capsules) — both indicated for prevention of recurrence after antibiotic treatment in adults with recurrent CDI

— Screening of donors: pathogens, multidrug-resistant organisms

— Adverse events: transient bloating, low risk of transmissible infection

— Indications: perforation, megacolon, necrotic bowel, end-organ failure, lactate ≥5, WBC ≥50,000, failure to improve within 3–5 days of medical therapy

— Procedures:

— Subtotal colectomy with end ileostomy — traditional standard

— Diverting loop ileostomy with intraoperative colonic lavage and postoperative vancomycin via ileostomy — colon-sparing alternative with comparable or better outcomes in select centers

CCS pearl: On CCS, after starting oral vancomycin and IV metronidazole for fulminant CDI, order surgical consultation in the same time block — delays in consulting surgery are a classic CCS scoring penalty.

Recurrence definition: return of symptoms within 8 weeks of completing therapy with confirmatory positive testing.

First recurrence treatment options:

Second or subsequent recurrence:

FMT details:

Surgical management for fulminant CDI:

Early surgical consultation saves lives — Step 3 emphasizes consulting before the patient is moribund.

Special Populations — Elderly and Renal/Hepatic Impairment

— Highest morbidity and mortality from CDI

— Higher recurrence rates → strongly consider fidaxomicin first-line and bezlotoxumab as adjunct

— More likely to present atypically: AMS, falls, anorexia rather than overt diarrhea

— Polypharmacy review: deprescribe PPIs, unnecessary antibiotics, antimotility agents

— Watch for AKI from volume depletion — gentle but adequate fluid resuscitation

— Functional assessment before discharge; many require subacute rehab

— Vancomycin PO and fidaxomicin require no renal dose adjustment — minimal systemic absorption

— Metronidazole: no dose adjustment needed for renal impairment, but accumulation of metabolites in ESRD with prolonged use may increase neurotoxicity risk

— Avoid nephrotoxins (NSAIDs, IV contrast when possible) during acute illness

— Watch for AKI as a marker of severity (Cr ≥1.5 = severe CDI)

— Metronidazole: reduce dose by 50% in severe hepatic impairment (Child-Pugh C)

— Vancomycin PO, fidaxomicin: no adjustment needed

— High colonization rates → strict testing criteria essential

— Outbreak control: cohort, contact precautions, soap and water hygiene, environmental cleaning with sporicidal agents (bleach 1:10)

— Antimicrobial stewardship reduces incidence

— Solid organ transplant, HSCT, chemotherapy, HIV, biologics: higher risk and worse outcomes

— Prefer fidaxomicin + consider bezlotoxumab

— Lower threshold for hospitalization

— FMT generally safe but screen donors rigorously; oral capsule products acceptable in many

Board pearl: In a 78-year-old nursing home resident with CDI and prior episode 4 months ago, the highest-yield answer is fidaxomicin plus bezlotoxumab — both age ≥65 and prior CDI within 6 months independently qualify for bezlotoxumab.

Elderly (age ≥65):

Renal impairment:

Hepatic impairment:

Long-term care residents:

Immunocompromised patients:

Special Populations — Pregnancy, Pediatrics, IBD

— CDI in pregnancy is uncommon but rising; associated with preterm labor, sepsis, and maternal mortality if fulminant

— Oral vancomycin is first-line in pregnancy — minimal systemic absorption, considered safe

— Fidaxomicin: limited data in pregnancy; reasonable if benefits outweigh risks, especially for recurrence

— Avoid metronidazole in the first trimester when possible (theoretical teratogenicity concern, though human data largely reassuring); acceptable later if needed

— Lactation: oral vancomycin and fidaxomicin compatible

— Bezlotoxumab: insufficient data; use only if benefit clear

— Infants <12 months: do NOT test routinely — high asymptomatic colonization (up to 70%); diarrhea almost always has another cause

— Children 1–2 years: test only after excluding other causes

— Children ≥2 years with risk factors: test as in adults

— Treatment:

— Non-severe: metronidazole or vancomycin (pediatric dosing) × 10 days

— Severe/fulminant: oral vancomycin ± IV metronidazole

— Fidaxomicin approved down to age 6 months

— Test for CDI at every flare — CDI both mimics and triggers flares

— Higher mortality and colectomy rates with CDI superinfection

— Vancomycin PO preferred initial therapy (some data favor over metronidazole in this group)

— Carefully balance immunosuppression escalation — many experts treat CDI for 48–72 hours before escalating biologics/steroids

— FMT effective for recurrent CDI in IBD, though small risk of IBD flare post-FMT

— Fidaxomicin preferred

— Bezlotoxumab valuable for recurrence prevention

— FMT generally safe with screened donors

Step 3 management: In a pregnant patient at 32 weeks with new CDI after cefazolin prophylaxis for a UTI, prescribe oral vancomycin 125 mg QID × 10 days, hold the cephalosporin, and arrange close OB follow-up.

Pregnancy and lactation:

Pediatrics:

Inflammatory bowel disease (IBD):

Oncology and HSCT:

Complications and Adverse Outcomes

— Toxic megacolon: colonic dilation >6 cm with systemic toxicity; mortality 30–50% if perforated

— Perforation: free air on imaging, peritonitis → emergent surgery

— Ileus: functional obstruction; impairs oral drug delivery → need rectal vancomycin

— Pseudomembranous colitis with severe mucosal damage

— Protein-losing enteropathy with hypoalbuminemia and anasarca

— Sepsis and septic shock with multiorgan failure

— AKI from volume depletion and sepsis

— Electrolyte derangements: hypokalemia, hypomagnesemia, metabolic acidosis (lactic from hypoperfusion, non-anion-gap from stool bicarbonate loss)

— DIC in fulminant disease

— Bacteremia is rare (CDI is toxin-mediated, not invasive); if blood cultures positive for C. difficile, look for secondary translocation or contamination

— 20–25% after first episode, 40–60% after second, ≥60% after third

— Risk factors: age ≥65, immunosuppression, continued antibiotics, PPI use, severe initial disease, prior recurrence

— Post-infectious IBS in up to 25%

— IBD unmasking or flare

— Prolonged colonization and shedding

— Functional decline in elderly — often the precipitant for transition to higher level of care

— Vancomycin PO: generally well tolerated; rare nephrotoxicity (consider checking levels only in fulminant disease with high doses + renal impairment)

— Fidaxomicin: nausea, vomiting; rare hypersensitivity

— Metronidazole: peripheral neuropathy, disulfiram reaction, neurotoxicity (encephalopathy, cerebellar dysfunction)

— Bezlotoxumab: CHF exacerbation, infusion reactions

— FMT: transient GI symptoms, rare transmission of pathogens

Key distinction: Ileus with worsening abdominal exam and rising lactate in a CDI patient on PO vancomycin is fulminant disease — the drug isn't reaching the colon. Switch to rectal vancomycin + IV metronidazole and call surgery, do not just continue PO therapy.

Local/colonic complications:

Systemic complications:

Recurrence:

Long-term sequelae:

Treatment-related complications:

When to Escalate Care — ICU, Consults, Inpatient Triage

— Hemodynamic instability requiring vasopressors

— Lactate ≥5 mmol/L or rising

— Respiratory failure (often from abdominal distension or sepsis)

— Altered mental status from sepsis

— Toxic megacolon or perforation

— WBC ≥50,000 (associated with very high mortality)

— Need for frequent monitoring during aggressive resuscitation

— Severe CDI (WBC ≥15,000 or Cr ≥1.5) but hemodynamically stable

— Unable to tolerate PO

— Significant comorbidity (HF, ESRD, immunocompromise)

— Social barriers to outpatient management

— Failed outpatient therapy

— Surgery: immediately for fulminant disease, megacolon, perforation, lactate ≥5, WBC ≥25,000, AMS, or failure to improve in 3–5 days

— Infectious disease: for recurrent CDI (≥2 episodes), unusual presentations, immunocompromised hosts, antibiotic stewardship

— Gastroenterology: for FMT consideration, IBD overlap, endoscopic evaluation

— Nephrology: persistent AKI

— Contact precautions with gown and gloves; continue ≥48 hours after diarrhea resolves

— Private room preferred; cohort if necessary

— Soap and water handwashing (alcohol-based sanitizer does NOT kill spores)

— Dedicated equipment (stethoscope, thermometer)

— Environmental cleaning with sporicidal agent (bleach or hydrogen peroxide-based)

— Notify environmental services and downstream units on transfer

CCS pearl: Time-sensitive CCS actions in fulminant CDI: vancomycin PO + metronidazole IV → IV fluids → surgical consult → contact precautions → CT abdomen/pelvis → serial lactate and WBC q4–6h. Move clock in short increments to catch deterioration.

ICU admission criteria:

Floor admission criteria:

Consultations:

Infection control measures (inpatient):

Antimicrobial stewardship engagement: review and de-escalate non-essential antibiotics; consult stewardship pharmacist.

Key Differentials — Other Infectious Diarrheas

— Salmonella (non-typhi): poultry, eggs, reptiles; fever, bloody or watery diarrhea; treat only if severe or high-risk host (fluoroquinolone, azithromycin, ceftriaxone)

— Shigella: low inoculum, daycare/MSM, tenesmus, dysentery; fluoroquinolone or azithromycin

— Campylobacter: poultry, unpasteurized milk; bloody diarrhea, associated with Guillain-Barré and reactive arthritis; azithromycin

— EHEC (O157:H7): undercooked beef, bloody diarrhea, no fever; avoid antibiotics (increase HUS risk); supportive care

— Yersinia: mesenteric adenitis mimicking appendicitis; pork

— Vibrio: seafood/seawater exposure

— Norovirus: cruise ships, outbreaks, vomiting prominent; supportive

— Rotavirus: pediatric, decreasing with vaccination

— Giardia: prolonged, foul-smelling, bulky, fatty stools, camping/well water; metronidazole or tinidazole

— Cryptosporidium: immunocompromised, watery diarrhea; nitazoxanide

— Entamoeba histolytica: travel, liver abscess; metronidazole + luminal agent

Board pearl: Bloody diarrhea makes CDI less likely as the sole diagnosis — think EHEC, Shigella, Salmonella, ischemic colitis, or IBD. Send stool culture and Shiga toxin testing alongside C. diff testing if bloody, and avoid empiric antibiotics until EHEC is excluded in suspicious cases.

Always consider non-CDI causes, especially in CA-CDI presentations or when stool testing is equivocal.

Bacterial enteritis:

Viral gastroenteritis:

Parasitic:

CMV colitis: immunocompromised, transplant, IBD on biologics; can coexist with CDI; biopsy for inclusions

C. perfringens food poisoning: large outbreaks, gravy/meat; short duration; supportive

Key Differentials — Non-Infectious Mimics

— Chronic course, extraintestinal manifestations (uveitis, erythema nodosum, arthritis, PSC)

— Endoscopy and biopsy differentiate; always test for CDI at flares — coinfection common

— Chronic watery diarrhea, older women, NSAID/PPI/SSRI association

— Normal-appearing colonoscopy → diagnosis on biopsy

— Treat: discontinue offending drug, budesonide

— Elderly, vasculopathy, hypotensive episode, postoperative; left-sided abdominal pain with bloody diarrhea

— CT: segmental wall thickening (often watershed: splenic flexure, rectosigmoid)

— Conservative management usually; surgery if full-thickness necrosis

— Focal LLQ pain, fever, leukocytosis; diarrhea less prominent

— CT confirms; antibiotics ± drainage/surgery

— Laxatives (most common confounder!), magnesium-containing antacids, metformin, colchicine, chemotherapy (irinotecan), checkpoint inhibitors (immune-mediated colitis), mycophenolate

— Checkpoint inhibitor colitis (anti-CTLA-4, anti-PD-1): treat with steroids; if refractory, infliximab or vedolizumab; rule out CDI first

— Osmotic, formula-related, fiber issues; common confounder for false-positive CDI testing

— Chronic, Rome IV criteria, no alarm features

— Post-infectious IBS can follow CDI

Key distinction: A hospitalized patient on continuous tube feeds with loose stools and a positive C. diff PCR but no leukocytosis, no fever, and no abdominal pain likely has tube feed–related diarrhea with C. diff colonization — adjust feed osmolality/rate and add soluble fiber before assuming CDI.

Inflammatory bowel disease (Crohn, ulcerative colitis):

Microscopic colitis (collagenous or lymphocytic):

Ischemic colitis:

Diverticulitis:

Medication-induced diarrhea:

Tube feed-associated diarrhea:

Irritable bowel syndrome (IBS):

Endocrine: hyperthyroidism, carcinoid, VIPoma, gastrinoma (rare)

Secondary Prevention and Discharge Planning

— Avoid unnecessary antibiotics in the future; document CDI history prominently in the chart

— When antibiotics are unavoidable, choose narrow-spectrum and shortest effective duration

— Counsel patient and family: never take leftover antibiotics, no antibiotics for viral URIs

— PPI: assess indication; switch to H2 blocker or stop if no clear indication (GERD without erosive disease, ulcer prophylaxis without risk factors)

— Reassess H2 blockers as well in low-risk patients

— Minimize antimotility agents, especially during active or recent CDI

— Bezlotoxumab during current treatment course in high-risk patients (age ≥65, prior CDI in 6 months, immunocompromised, severe disease)

— Fidaxomicin preferred over vancomycin if either available

— Consider fidaxomicin extended-pulsed or vancomycin taper in recurrent cases

— FMT after ≥2 recurrences; FDA-approved products (Rebyota, Vowst) for prevention of recurrence

— Complete full antibiotic course even if symptoms resolve

— Hand hygiene with soap and water at home (especially after toilet use)

— Clean bathroom surfaces with bleach-based products

— No need to discard household items; routine laundry adequate

— Avoid food preparation for others until 48h asymptomatic

— Return precautions: recurrent diarrhea, fever, severe abdominal pain, lightheadedness

Step 3 management: At discharge after a first severe CDI episode in a 72-year-old, the high-yield orders are: stop the home PPI (unless strong indication), document a severe penicillin/cephalosporin/fluoroquinolone caution in the problem list, schedule PCP follow-up within 1 week, and provide written instructions on when to return.

Antimicrobial stewardship — the most important prevention lever:

Deprescribe risk-amplifying medications:

Recurrence prevention strategies:

Counseling at discharge:

Vaccination considerations: there is currently no FDA-approved vaccine; investigational toxoid vaccines in trials.

Follow-Up, Monitoring, and Counseling

— Phone or telehealth check at 48–72 hours after starting therapy: confirm clinical improvement (stool frequency decreasing, pain improving, tolerating PO)

— In-person or telehealth visit at end of 10-day course to confirm resolution

— Follow-up at 2 and 8 weeks post-treatment to assess for recurrence (highest risk window)

— For recurrent CDI patients, consider closer monitoring and earlier GI/ID referral

— Symptom resolution: stool frequency, consistency, abdominal pain, appetite, weight

— Hydration and functional status, especially in elderly

— Repeat labs if persistent symptoms: BMP for AKI/electrolytes, CBC for ongoing leukocytosis

— Do NOT obtain test of cure — up to 50% remain PCR-positive after successful treatment; testing only confounds care

— Repeat testing only if new symptoms recur after initial resolution

— Resume normal diet as tolerated; no specific restrictions

— Adequate hydration; oral rehydration solutions for elderly or volume-depleted

— Probiotics not routinely recommended (insufficient evidence; risk in immunocompromised); do not discourage if patient already takes

— Gradual return to activity; rehab assessment if deconditioned

— Add "C. difficile infection — high recurrence risk" to chart problem list

— Educate patient: tell every provider about CDI history, especially before dental or surgical procedures

— If future antibiotics required, consider co-administering oral vancomycin prophylaxis (125 mg daily or BID during the antibiotic course) in patients with multiple prior recurrences — emerging practice, not yet guideline-strong

Board pearl: Never order a "test of cure" stool study after CDI treatment in an asymptomatic patient — this is a classic Step 3 distractor. Clinical resolution defines cure.

Post-treatment follow-up cadence:

What to monitor:

Diet and lifestyle counseling:

Antibiotic future-use planning:

Mental health and quality of life: chronic recurrent CDI is associated with anxiety, social isolation, and depression — screen and refer.

Ethical, Legal, and Patient Safety Considerations

— Inappropriate antibiotic use is the leading modifiable risk factor for CDI

— Institutions are accountable for CDI rates as a publicly reported quality metric (CMS hospital-acquired condition; affects reimbursement)

— Document indication and planned duration on every antibiotic order

— CDI is reportable in many jurisdictions (varies by state); healthcare-associated CDI rates are reported to NHSN

— Outbreak recognition triggers public health investigation

— Highest-risk transitions: hospital → SNF/LTC; SNF → home; hospital → home with home health

— Failure to communicate CDI status, contact precaution needs, and active treatment regimen at handoff leads to outbreaks and recurrences

— Use structured handoff tools; include CDI status, day of treatment, planned end date, and antibiotic stewardship notes in discharge summary

— Confirm receiving facility/pharmacy has the prescribed agent (fidaxomicin and bezlotoxumab often require prior authorization — anticipate and start the paperwork before discharge)

— FMT: discuss alternative therapies, infection transmission risk (small but real, including MDROs), unknown long-term effects, off-label routes; written consent recommended

— Colectomy for fulminant CDI in obtunded patient: surrogate consent; involve family early; use advance directives; ethics consult if conflict

— Hand hygiene with soap and water (alcohol gel does not kill spores) is non-negotiable — protect roommates, staff, future patients

— Visitor education

— Fidaxomicin and bezlotoxumab costs create access disparities; engage social work and pharmacy benefits navigation

— Nursing home residents disproportionately affected — facility-level stewardship is an equity intervention

Step 3 management: A patient with fulminant CDI in septic shock who is intubated and sedated needs urgent colectomy; obtain consent from the healthcare power of attorney per the patient's prior advance directive, and document the surrogate discussion. If no surrogate available and time is critical, proceed under emergency doctrine with two-physician documentation.

Antimicrobial stewardship as a patient safety imperative:

Public health reporting:

Transition-of-care risks:

Informed consent edge cases:

Infection control compliance:

Equity considerations:

High-Yield Associations and Rapid-Fire Clinical Facts

Board pearl: A "test of cure" stool PCR is wrong on the test, wrong in practice, and a classic distractor — clinical assessment defines cure.

Highest-risk antibiotics: clindamycin, fluoroquinolones, 3rd/4th-generation cephalosporins, carbapenems, broad-spectrum penicillins.

Lowest-risk antibiotics: aminoglycosides, daptomycin, tigecycline, metronidazole, vancomycin (oral), nitrofurantoin.

Severity cutoffs: WBC ≥15,000 OR Cr ≥1.5 mg/dL = severe; hypotension, ileus, or megacolon = fulminant.

First-line therapy (2021 IDSA): fidaxomicin preferred; vancomycin acceptable; metronidazole only if alternatives unavailable.

Fulminant CDI: PO vancomycin 500 mg QID + IV metronidazole 500 mg q8h; add PR vancomycin if ileus.

Recurrence rates: 20–25% after 1st, 40–60% after 2nd, ≥60% after 3rd.

FMT efficacy: 80–90% cure rate for recurrent CDI.

Bezlotoxumab criteria: age ≥65, prior CDI in 6 months, immunocompromised, severe disease — single IV infusion with antibiotics.

Hand hygiene: soap and water, NOT alcohol-based sanitizers (spores resist alcohol).

Environmental cleaning: bleach (1:10) or EPA-registered sporicidal agents.

Toxic megacolon criteria: colonic dilation >6 cm + systemic toxicity (fever, tachycardia, leukocytosis, anemia) + ≥1 of (dehydration, AMS, electrolyte abnormality, hypotension).

Surgical indications: perforation, megacolon, lactate ≥5, WBC ≥50,000, AMS, failure to improve in 3–5 days.

Surgical procedure: subtotal colectomy with end ileostomy OR diverting loop ileostomy with colonic lavage.

Do NOT test: asymptomatic patients, formed stool, after recent treatment (test of cure), within 7 days of a previous test, infants <12 months.

Avoid: loperamide and other antimotility agents in moderate-severe disease; IV vancomycin (ineffective).

PPI association: modest but real increased risk — deprescribe if no firm indication.

IBD overlap: test for CDI at every flare; vancomycin PO preferred.

Board Question Stem Patterns

— "A 68-year-old woman 10 days post dental clindamycin develops watery diarrhea, WBC 12,000, Cr 1.0."

— Answer: Fidaxomicin 200 mg PO BID × 10 days (non-severe; fidaxomicin preferred over vancomycin).

— "Post-op patient with sudden cessation of diarrhea, distended abdomen, HR 130, lactate 4.0, KUB shows 8-cm transverse colon."

— Answer: PO/NG vancomycin 500 mg QID + IV metronidazole + PR vancomycin + urgent surgical consult, not "continue current therapy."

— "Asymptomatic patient with one loose stool, on lactulose, NAAT positive."

— Answer: Do not treat; reassess clinical criteria.

— "Patient completed vancomycin 3 weeks ago; new diarrhea, NAAT positive."

— Answer: Fidaxomicin (standard or extended-pulsed) ± bezlotoxumab if high-risk.

— "Third CDI episode in 8 months despite vancomycin and fidaxomicin courses."

— Answer: Fecal microbiota transplantation (or FDA-approved Rebyota/Vowst after standard antibiotic course).

— "Best method to prevent transmission?"

— Answer: Soap and water, contact precautions, sporicidal cleaning.

— "UC patient with flare and bloody diarrhea." First step: stool C. diff testing before escalating immunosuppression.

— Answer: Oral vancomycin, not metronidazole (especially 1st trimester).

— Distractors: IV vancomycin (no role), loperamide (dangerous), probiotics alone (insufficient evidence).

— Failure to improve in 3–5 days, lactate ≥5, WBC ≥50,000 → answer is surgical consultation/colectomy, not "continue antibiotics 5 more days."

Step 3 management: When a stem mentions both severity criteria AND hemodynamic instability or ileus, default to fulminant CDI management — high-dose oral vancomycin, IV metronidazole, rectal vancomycin if ileus, and immediate surgical consultation.

Pattern 1 — The post-antibiotic outpatient:

Pattern 2 — The ileus trap:

Pattern 3 — The colonization trap:

Pattern 4 — The recurrence:

Pattern 5 — The multiply-recurrent patient:

Pattern 6 — The hand hygiene question:

Pattern 7 — The IBD flare:

Pattern 8 — The pregnant patient:

Pattern 9 — The wrong drug:

Pattern 10 — The surgical timing:

One-Line Recap

— Non-severe: WBC <15k AND Cr <1.5 → fidaxomicin or PO vancomycin

— Severe: WBC ≥15k OR Cr ≥1.5 → fidaxomicin or PO vancomycin

— Fulminant: hypotension, ileus, megacolon → PO vancomycin 500 mg QID + IV metronidazole ± PR vancomycin + surgery

— 1st recurrence: fidaxomicin (or vancomycin taper) ± bezlotoxumab if high-risk

— ≥2nd recurrence: FMT or FDA-approved microbiome products (Rebyota, Vowst)

— Treating asymptomatic NAAT-positive colonization

— Ordering a "test of cure" stool study

— Continuing PO vancomycin alone in a patient with ileus or worsening fulminant disease instead of escalating to rectal vancomycin + IV metronidazole + early surgical consultation

Board pearl: If you remember nothing else — fidaxomicin is now first-line, IV vancomycin doesn't work, soap and water beats alcohol gel, and fulminant disease needs surgery on speed-dial, not just more antibiotics.

Bottom line: C. difficile infection is a toxin-mediated colitis triggered by antibiotic-induced dysbiosis — diagnose with stool toxin testing only in symptomatic patients with ≥3 unformed stools/24h, stratify by WBC, Cr, and hemodynamics, and treat first-line with fidaxomicin (or vancomycin) for non-severe/severe disease and high-dose oral vancomycin plus IV metronidazole (with rectal vancomycin and surgical consult) for fulminant disease.

Severity cheat sheet:

Recurrence pathway:

Top three Step 3 traps to avoid:

Prevention pillars: antimicrobial stewardship, PPI deprescribing, soap-and-water hand hygiene, sporicidal environmental cleaning, contact precautions, and structured handoffs at every transition of care.