Diarrhea
Acute <14 d, mostly viral. Workup if severe, bloody, high-risk, >1 week: BMP, C. diff, stool PCR, O&P if travel/MSM/immunocompromised.
Hailey Harris5 min6 questions1 trials
Acute diarrhea — most of it is viral
≥3 loose stools/day for <14 days (ACG: ; : ; ). Small-bowel diarrhea is watery, large-volume, crampy/bloating. Large-bowel is frequent, small-volume, painful, ± fever, blood, mucus.
Work up
- High-risk: immunocompromised / HIV, IBD, age >70, pregnancy.
- Severe: fever >101.3°F, >6 BMs/24 h, hypovolemia, severe pain.
- BMP, C. diff, stool PCR if high-risk, severe, bloody, or >1 week.
- O&P × 3 q24 h if travel, MSM, or immunocompromised. Bloody stool: Shiga toxin + fecal leukocytes.
Do not
- Antibiotics if STEC is on the table (↑ HUS).
- Retest C. diff within 7 days of a positive.
- Treat a PCR colonization — correlate clinically.
- Probiotics as standard of care.
- Exposure history: travel, antibiotics, hospital, food, sick contacts, daycare.
- Immunocompromised extras: CMV, C. diff, Cryptosporidium, microsporidia, MAC, TB, Histoplasma.
- Volume and electrolytes first. Empiric FQ or azithromycin only if severe, high-risk, septic, bloody, age ≥70, or serious comorbidity — azithromycin preferred for Asia / Campylobacter.
- C. diff: PO vancomycin or fidaxomicin.
Common pathogens
| Organism | Clue |
|---|---|
| Norovirus | Winter outbreaks; nausea/vomiting prominent |
| Rotavirus | Daycare |
| Adenovirus | Conjunctivitis + pharyngitis |
| ETEC / STEC | Traveler’s diarrhea vs undercooked hamburger + Shiga toxin / HUS |
| Campylobacter | Undercooked/unpasteurized; reactive arthritis, GBS |
| Salmonella | Eggs, poultry, milk; bacteremia is common |
| Shigella | Tiny inoculum; often hematochezia |
| Vibrio | Shellfish / salt water; severe in cirrhosis |
| Yersinia | Undercooked pork; pseudo-appendicitis (RLQ) |
| Giardia | Streams; watery → greasy malabsorptive |
| Crypto / Cystoisospora | Water outbreaks; contaminated produce |
| E. histolytica | Food/water outside the US; liver abscess |
Chronic and persistent
Persistent 14–30 days; chronic ≥4 weeks (; ). Alarm: onset >50, bleeding, nocturnal pain/diarrhea, progressive pain, unexplained fever or weight loss → colonoscopy.
- Labs: CBC, BMP, LFTs, TSH; stool Na/K/pH, calprotectin or lactoferrin, elastase, 24–48 h fecal fat, FOBT, Giardia, celiac serologies ().
- Negative calprotectin / lactoferrin essentially rules out IBD. Positive FOBT with diarrhea points to chronic infection or IBD.
- Osmotic gap = 290 − 2(stool Na + K). Normal 50–100 mOsm/kg. >125 osmotic; <50 secretory.
| Pattern | Gap | Fasting | Think | Next |
|---|---|---|---|---|
| Secretory | <50 | No change; often nocturnal | VIPoma, microscopic colitis, bile salt, villous adenoma, hyperthyroid, Addison, mastocytosis, DM neuropathy, amyloid | Exclude infection; colo + bx; chromogranin, gastrin, 5-HIAA, ACTH stim as indicated. Cholestyramine 4 g; budesonide for microscopic colitis; octreotide for VIP |
| Osmotic | >125 | Improves | Lactose, mannitol/sorbitol, magnesium, laxatives, FODMAPs | Stool pH <6, H2 breath test, laxative screen; stop the agent |
| Functional | 50–100 | Variable | IBS, functional diarrhea | Fiber, eluxadoline if pain, hyoscyamine, rifaximin if bloating, TCA/SSRI |
| Fatty / maldigestive | Usually >50 | Improves | Pancreatic insufficiency, bile-acid loss, SIBO, short gut, mucosal disease, mesenteric ischemia | Sudan / 24 h fat (>20 g pancreatic, 14–20 g small bowel); elastase. Pancrelipase 500–2500 u/kg/meal; cholestyramine post-cholecystectomy |
| Inflammatory | Usually <50 | No change | IBD, C. diff, ameba, Yersinia, TB, CMV/HSV, colon CA, ICI colitis, radiation | Calprotectin, leukocytes, colo + bx; antimicrobial vs immunosuppression |