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Mr. C is a 35-year-old man who comes to your outpatient office complaining of 1 day of diarrhea.

Image not available.What is the differential diagnosis of diarrhea? How would you frame the differential?

Although the presence of diarrhea is actually defined by stool weight, it is more useful to define acute diarrhea clinically. Diarrhea can be thought of as bowel movements of a looser consistency than usual that occur more than 3 times a day. Acute diarrhea develops over a period of 1–2 days and lasts for less than 4 weeks. (This chapter will not address chronic or intermittent diarrhea.) The differential diagnosis below uses the pivotal point of presenting symptoms to organize diagnoses into three categories: noninfectious, gastroenteritis, and infectious colitis. This structure is easy to remember, focuses history taking, allows prognosticating, and is also a good framework on which to consider therapy See Fig. 12–1.


Noninfectious diarrhea is recognized by the lack of constitutional symptoms. Infectious diarrhea that presents with large volume (often watery) stool, constitutional symptoms, nausea and vomiting, and often abdominal cramps can be categorized as gastroenteritis. Infectious colitis presents with fever, tenesmus, and dysentery (stools with blood and mucus). Many organisms can cause both gastroenteritis and inflammatory diarrhea.


  1. Noninfectious diarrhea

    1. Medications and other ingestible substances (some with osmotic effect)

      1. Sorbitol (gum, mints, pill fillers)

      1. Mannitol

      1. Fructose (fruits, soft drinks)

      1. Fiber (bran, fruits, vegetables)

      1. Lactulose

    1. Magnesium-containing medications

      1. Nutritional supplements

      1. Antacids

      1. Laxatives

    1. Malabsorption

      1. Lactose intolerance

      1. Pancreatitis

    1. Medications causing diarrhea through nonosmotic means

      1. Metformin

      1. Antibiotics

      1. Colchicine

      1. Digoxin

      1. Selective serotonin reuptake inhibitor antidepressants

  2. Infectious diarrhea: gastroenteritis

    1. Viral (most common)

      1. Caliciviruses (Norovirus, formally Norwalk virus)

      1. Rotovirus

    1. Bacterial (commonly food-borne)

      1. Vibrio cholera

      1. Escherichia coli

      1. Shigella species

      1. Salmonella species

      1. Campylobacter species

      1. Yersinia enterocolitica

    1. Toxin-mediated

      1. Staphylococcus aureus

      1. Clostridium perfringens

      1. Bacillus cereus

      1. E coli

  3. Infectious diarrhea: inflammatory colitis

    1. Bacterial

      1. Shigella species

      1. E coli

      1. Campylobacter species

      1. Salmonella species

      1. Y enterocolitica

    1. Antibiotic-associated

      1. Clostridium difficile

      1. Klebsiella oxytoca

      1. Non-C difficile–related

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The first symptom the patient noted was a poor appetite while eating breakfast. He was unable to finish his usual cup of coffee and a bowl of cereal. During his 20-minute drive to work he developed nausea and diaphoresis. Upon arriving at work he developed low-grade fever, abdominal cramping, and vomiting. Over the next 12 hours, diarrhea developed. He describes the stool being watery and brown without any blood.

Image not available.At this point, what is the leading hypothesis, what are the active alternatives, and is there a must not miss diagnosis? Given this differential diagnosis, what tests should be ordered?

Mr. C seeks medical attention within about 24 hours of the onset of diarrhea. The pivotal points in his history are acute onset of symptoms over about 60 ...

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