The essentials

Diarrhea means loose, watery stools three or more times a day. Most bouts come from a stomach bug, food poisoning or a medicine and pass within days. Diarrhea lasting 4 weeks or more, or with blood, fever or weight loss, calls for stool and blood tests.

Doctors sort diarrhea by how long it lasts. Acute diarrhea usually clears in less than a week, persistent diarrhea lasts 2 to 4 weeks, and chronic diarrhea lasts at least 4 weeks, steadily or on and off[1]. In the United States, up to 5% of people have chronic diarrhea[1]. On the Bristol stool chart, loose stools are types 5 to 7; frequent but formed stools are not diarrhea[9][5].

When is diarrhea an emergency?

When to see a doctor

  • Emergency: call 911 for confusion, fainting, almost no urine, black and tarry stools or a lot of red blood, or severe belly or rectal pain[2].
  • Within 24 to 48 hours for a fever above 101°F, blood or pus in the stool, repeated vomiting, six or more loose stools a day, diarrhea after travel abroad, or diarrhea that started on or after antibiotics[2][4][10].
  • At your next appointment if an adult is not better after 5 days[4], or if loose stools have come and gone for 4 weeks or more[8].

Pregnancy, age over 65, a weak immune system or current antibiotics raise the risk of complications: keep your doctor informed[1]. Thirst, dark urine, dizziness and less urine are early signs of dehydration[2].

What causes sudden diarrhea? The usual suspects first

  • Common Viral stomach bugs. Norovirus, rotavirus, adenovirus and astrovirus top the list of viral gastroenteritis causes[2].
  • Common Food poisoning. Salmonella, Campylobacter, E. coli and staph are frequent culprits, from undercooked meat, raw milk, or food and water while traveling. Most cases end within a week[2].
  • Common Medicines. Antibiotics, magnesium antacids, cancer drugs and syrups sweetened with sorbitol or xylitol[2].
  • Possible C. difficile. After antibiotics, this bacterium can overgrow in the colon: 5 to 10 watery stools a day, cramps, fever. Any antibiotic can trigger it[10].

Why does diarrhea last for weeks?

  • Common Irritable bowel syndrome and functional diarrhea. Diagnosed when basic tests are normal and there are no alarm features[8].
  • Common Sugars the gut cannot handle. Lactose, fructose and sugar alcohols; this trouble can also linger after an infection[2].
  • Possible Celiac disease, which every adult with chronic diarrhea should be tested for[8].
  • Possible Bile acid diarrhea. Found in up to 30% of people labeled with diarrhea-predominant IBS; it is behind much of the diarrhea that follows gallbladder removal[8][9].
  • Possible Microscopic colitis. Only biopsies show it; NSAID painkillers, PPI acid reducers and sertraline are linked to it[9].
  • Possible Lingering infections. Giardia and other intestinal parasites, or C. difficile[2][7].
  • Rare Inflammatory bowel disease (Crohn's disease, ulcerative colitis), an overactive thyroid, pancreas problems and colorectal cancer. Weight loss, anemia, persistent blood in the stool or a family history of these diseases calls for prompt testing, often a colonoscopy[7][8].

Which stool and blood tests will the doctor order?

A short, mild bout needs no test. Stool tests are for fever, bloody or mucus-filled stools, severe cramps, sepsis, recent antibiotics or a hospital stay[6], and diarrhea that lasts. Blood tests look for dehydration and for diseases that cause diarrhea[3].

Tests commonly used to investigate diarrhea in adults
TestWhat it answers
Basic metabolic panel (sodium, potassium, BUN, creatinine)Have you lost too much fluid and salts?
Complete blood count and CRPIs there anemia or inflammation?
tTG-IgA with total IgACould it be celiac disease?
TSH, ferritinThyroid disorder or iron deficiency?
GI pathogen PCR panel or stool cultureWhich germ causes an infection with fever or blood?
C. difficile toxin or PCRIs diarrhea after antibiotics or a hospital stay due to C. difficile?
Giardia antigen or PCRIs a parasite involved?
Fecal calprotectin or lactoferrinIs the bowel lining inflamed?

For chronic diarrhea, US family physicians start with a CBC, CRP, basic metabolic panel, tTG-IgA and total IgA, then add TSH or others based on symptoms[8]; British guidance also lists ferritin[9]. US gastroenterologists prefer calprotectin or lactoferrin over CRP or ESR to screen for inflammation, and a targeted Giardia test over a routine ova and parasites exam unless you have traveled[7]. A hydrogen breath test checks for sugar intolerances[3]. See our guide to stool test types.

Results back after weeks of loose stools? BloodSense explains your whole report line by line, in plain words, so you know what to ask your doctor.

Explain my results

How do I read the results?

Fictitious example of a lab report after 5 weeks of diarrhea (United States format)
TestResultReference rangeFlag
Sodium137 mEq/L135-145 mEq/L
Potassium3.2 mEq/L3.5-5.1 mEq/LL
BUN24 mg/dL7-20 mg/dLH
Creatinine0.9 mg/dL0.6-1.2 mg/dL
Hemoglobin13.8 g/dL12.0-15.5 g/dL
tTG-IgA1.1 U/mL< 4.0 U/mL
Giardia antigen (stool)NegativeNegative
Fecal calprotectin31 µg/g< 50 µg/g
  1. L means below the lab's range. Fluid lost through diarrhea carries potassium away; a low level can cause muscle cramps, weakness and an irregular heartbeat[11].
  2. H means above the range. Dehydration is one reason BUN goes up[12]; your doctor reads it next to creatinine (see the BUN to creatinine ratio).
  3. The other lines are normal: anemia, celiac disease, Giardia and bowel inflammation become less likely.

Calprotectin: inflamed bowel or not?

Below 50 µg/g, inflammatory bowel disease is unlikely; 50 to 249 is a gray zone; above 250 points to active inflammation, though cut-offs vary by assay[9]. A low value does not rule out IBD when blood, weight loss or anemia are present[8].

Infection tests: detected or not detected

PCR can find a germ's genes without active toxin, so a positive C. difficile PCR is read with your symptoms and often a toxin test[9]. For C. difficile, one stool sample is usually enough[6].

Bile acids: limited testing in the US

The SeHCAT scan is not FDA-approved; a fasting C4 blood test is used where available[8], or a trial of a bile acid binder[7].

What should I note before the appointment?

  • The start date, and whether it is constant or comes and goes
  • Stools per day, their Bristol type, and whether they wake you at night
  • Blood, mucus, black color, or greasy stools that float (see yellow, greasy stool)
  • Weight change, fever, urine amount
  • New medicines, recent antibiotics, supplements, alcohol
  • Travel, sick contacts, family history, past bowel or gallbladder surgery

Frequently asked questions

Can I take loperamide or bismuth for diarrhea?

For most adults with acute diarrhea, yes, for a short time. Doctors advise against them with bloody stools or fever, and you should see a doctor if diarrhea worsens or lasts more than 2 days on them[13]. Drinking enough, ideally an oral rehydration solution, matters most[5].

Does diarrhea at night mean something serious?

Not always, but it is worth reporting. Watery diarrhea that continues at night or while fasting points doctors toward a secretory cause, such as bile acid loss or microscopic colitis, rather than IBS[8].

Do I need a colonoscopy for chronic diarrhea?

Not always. It is done when there are alarm features, abnormal blood or stool results, or no answer after the first tests. Biopsies are taken even if the colon looks normal, which is how microscopic colitis is found[8].

References

  1. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Definition & facts for diarrhea. niddk.nih.gov (accessed November 2, 2026)
  2. NIDDK. Symptoms & causes of diarrhea. niddk.nih.gov (accessed November 2, 2026)
  3. NIDDK. Diagnosis of diarrhea. niddk.nih.gov (accessed November 2, 2026)
  4. MedlinePlus Medical Encyclopedia. Diarrhea. medlineplus.gov (accessed November 2, 2026)
  5. World Health Organization. Diarrhoeal disease: fact sheet. who.int (accessed November 2, 2026)
  6. Shane AL, Mody RK, Crump JA, et al. 2017 Infectious Diseases Society of America clinical practice guidelines for the diagnosis and management of infectious diarrhea. Clin Infect Dis. 2017;65(12):e45-e80. doi:10.1093/cid/cix669 (accessed November 2, 2026)
  7. Smalley W, Falck-Ytter C, Carrasco-Labra A, Wani S, Lytvyn L, Falck-Ytter Y. AGA clinical practice guidelines on the laboratory evaluation of functional diarrhea and diarrhea-predominant irritable bowel syndrome in adults (IBS-D). Gastroenterology. 2019;157(3):851-854. doi:10.1053/j.gastro.2019.07.004 (accessed November 2, 2026)
  8. Burgers K, Lindberg B, Bevis ZJ. Chronic diarrhea in adults: evaluation and differential diagnosis. Am Fam Physician. 2020;101(8):472-480. aafp.org (accessed November 2, 2026)
  9. Arasaradnam RP, Brown S, Forbes A, et al. Guidelines for the investigation of chronic diarrhoea in adults: British Society of Gastroenterology, 3rd edition. Gut. 2018;67(8):1380-1399. doi:10.1136/gutjnl-2017-315909 (accessed November 2, 2026)
  10. MedlinePlus Medical Encyclopedia. Pseudomembranous colitis. medlineplus.gov (accessed November 2, 2026)
  11. MedlinePlus Lab Tests. Potassium blood test. medlineplus.gov (accessed November 2, 2026)
  12. MedlinePlus Lab Tests. BUN (blood urea nitrogen). medlineplus.gov (accessed November 2, 2026)
  13. NIDDK. Treatment of diarrhea. niddk.nih.gov (accessed November 2, 2026)

This article is general health information. It does not replace a medical examination, diagnosis or treatment. With signs of dehydration, blood in the stool, high fever or severe pain, seek care without waiting.