DIARRHEA

>3 loose stools/day or increased frequency from baseline — acute (<4 weeks) vs. chronic; infectious vs. non-infectious

SYMPTOMS / ASSOCIATED SX

  • Stool frequency, consistency, blood/mucus, odor, floating (fat malabsorption)

  • Crampy abdominal pain (infectious colitis, IBD); tenesmus (rectal involvement)

  • Fever, chills (infectious — C. diff, invasive bacteria, viral gastroenteritis)

  • Volume depletion: thirst, orthostasis, decreased urine output

  • Recent antibiotic use (C. difficile); hospitalization; immunocompromised state; travel history

DENIES

  • Hematochezia/melena (GI bleeding — infectious colitis, ischemic colitis, IBD)

  • Fever >38.5°C (infectious — C. diff, bacterial colitis)

  • Recent antibiotic use in last 3 months (C. diff primary risk factor)

  • Immunocompromised state (opportunistic infections — CMV, Cryptosporidium)

SOCIAL HISTORY

  • Recent travel (traveler's diarrhea — ETEC most common); sick contacts; food exposures (shellfish, undercooked meat)

  • Antibiotic use in last 3 months; hospitalization in last 12 months; nursing home residence (C. diff risk)

  • Alcohol (direct GI irritant); dietary changes

MAIN ETIOLOGY

  • Infectious: C. difficile (post-antibiotic, hospitalized — most important inpatient); viral (norovirus, rotavirus); bacterial (Salmonella, Shigella, Campylobacter, ETEC, E. coli O157:H7)

  • Non-infectious: medications (metformin, SSRIs, antibiotics, PPIs, NSAIDs, colchicine, Mg-containing antacids), malabsorption (celiac, pancreatic EPI), osmotic (lactulose, sorbitol)

  • Inflammatory: IBD (Crohn's, UC), ischemic colitis, radiation colitis

MOST COMMON DDX

  • C. difficile (positive PCR/GDH/toxin; post-antibiotic; pseudomembranes on colonoscopy)

  • Viral gastroenteritis (norovirus; no blood; self-limited)

  • IBD flare (bloody diarrhea, abdominal pain, elevated CRP/fecal calprotectin; colonoscopy)

  • Ischemic colitis (acute abdominal pain + bloody diarrhea; elderly/vascular disease; CT colon)

  • Celiac disease (chronic, steatorrhea; anti-tTG IgA)

  • Overflow diarrhea from fecal impaction (rectal exam — hard stool)

  • Medication-related (new medications within 4–6 weeks)

DATA

  • C. diff toxin PCR (test all inpatient diarrhea ≥3 loose stools; do not test formed stool)

  • Stool cultures (if fever, bloody diarrhea, immunocompromised, travel)

  • Stool O&P ×3 (chronic, travel, immunocompromised)

  • Fecal occult blood; fecal WBCs/lactoferrin (inflammatory); fecal calprotectin (IBD)

  • BMP (electrolytes, renal function); CBC (leukocytosis — C. diff, sepsis)

  • CRP, ESR; procalcitonin (bacterial sepsis)

  • CT abdomen/pelvis (toxic megacolon, IBD, ischemic colitis, perforation)

HOME MEDS

  • Antibiotics (last 3 months) — most common C. diff cause; if active, change to lowest-risk antibiotic

  • Metformin, SSRIs, NSAIDs, colchicine — hold/switch if causative

  • Loperamide — HOLD if C. diff suspected or bloody diarrhea (risk of toxic megacolon)

PLAN

  • Hydration and electrolyte replacement: IV fluids if unable to maintain oral or hemodynamically unstable

  • C. difficile (IDSA 2021):

    • Non-severe (WBC <15k, Cr <1.5): vancomycin 125 mg PO QID ×10 days OR fidaxomicin 200 mg PO BID ×10 days (lower recurrence rate — DIFICID trial)

    • Severe (WBC ≥15k or Cr ≥1.5): vancomycin 500 mg PO QID ×10 days

    • Fulminant (ileus, toxic megacolon, hypotension): vancomycin 500 mg PO/NGT QID + metronidazole 500 mg IV q8h; surgery consult

    • First recurrence: vancomycin taper/pulse regimen or fidaxomicin; bezlotoxumab 10 mg/kg IV for high-recurrence-risk patients

    • Contact precautions; soap and water handwashing (alcohol gel ineffective for C. diff spores)

  • Bacterial infectious diarrhea:

    • Most self-limited; supportive care

    • Shigella: ciprofloxacin 500 mg PO BID ×3 days or azithromycin 500 mg PO daily ×3 days

    • Campylobacter: azithromycin 500 mg PO daily ×3 days

    • E. coli O157:H7 (STEC): NO antibiotics — increases HUS risk; supportive care only

    • Salmonella: treat immunocompromised, typhoid fever, or bacteremia

  • IBD flare: GI consult; mesalamine for mild UC; prednisone 40–60 mg PO daily for moderate-severe

  • Anti-motility: loperamide 4 mg PO then 2 mg after loose stool (max 16 mg/day) — non-infectious/watery traveler's diarrhea ONLY; contraindicated in C. diff, STEC, bloody diarrhea

  • DISCHARGE:

    • C. diff: complete full antibiotic course; no test-of-cure if asymptomatic; counsel on hygiene

    • IBD: GI follow-up; continue immunosuppression

    • Avoidance of trigger antibiotics; PCP follow-up

RED FLAGS

  • Fever >38.5 + bloody diarrhea + leukocytosis → toxic megacolon risk; CT abdomen/pelvis; surgery consult

  • Toxic megacolon: colonic dilation >6 cm + systemic toxicity → emergent surgery; IV vanc + metro + NGT

  • E. coli O157:H7 + bloody diarrhea → NO antibiotics; HUS risk (thrombocytopenia + AKI + MAHA)

  • Immunocompromised + diarrhea → C. diff, CMV colitis, Cryptosporidium; colonoscopy + biopsy

SENIOR IM RESIDENT PEARLS

  • C. diff PCR is very sensitive — only test patients with ≥3 loose stools; do not test solid stool (false-positive context)

  • Fidaxomicin (DIFICID trial): lower recurrence rate than vancomycin in non-ribotype 027 C. diff — preferred when recurrence risk is high

  • C. diff spores resistant to alcohol-based hand sanitizer — must use soap and water; critical for infection control

  • Common mistake: giving loperamide to C. diff or bloody diarrhea — risk of toxic megacolon

  • STEC (E. coli O157:H7): antibiotics increase Shiga toxin release → HUS (NEJM 2000); supportive care only

  • FMT (fecal microbiota transplant): >2 C. diff recurrences; ~90% cure rate; refer to GI

  • Common mistake: attributing all inpatient diarrhea to "tube feeds" — always test for C. diff first