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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 What is the differential diagnosis of diarrhea? How would you frame the differential?


Clinically, it is probably most useful to define acute diarrhea as diarrhea (bowel movements of a looser consistency than usual that occur more than 3 times a day) that develops over a period of 1–2 days and lasts for less than 4 weeks. The differential diagnosis below uses pivotal points in a patient’s presenting symptoms to organize potential diagnoses into 3 categories: noninfectious, gastroenteritis, and infectious colitis. 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). This structure is easy to remember, focuses history taking, allows prognosticating, and is also a good framework on which to consider therapy. Figure 13-1 uses this framework to suggest therapy.

Figure 13-1.
Diagnostic approach: diarrhea.

  1. Noninfectious diarrhea

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

      1. Sorbitol (gum, mints, pill fillers)

      2. Mannitol

      3. Fructose (fruits, soft drinks)

      4. Fiber (bran, fruits, vegetables)

      5. Lactulose

    2. Magnesium-containing medications

      1. Nutritional supplements

      2. Antacids

      3. Laxatives

    3. Malabsorption

      1. Lactose intolerance

      2. Pancreatitis

    4. Medications causing diarrhea through nonosmotic means

      1. Metformin

      2. Antibiotics

      3. Colchicine

      4. Digoxin

      5. Selective serotonin reuptake inhibitor antidepressants

  2. Infectious diarrhea: gastroenteritis

    1. Viral (most common)

      1. Caliciviruses (including Norovirus)

      2. Rotovirus

    2. Bacterial

      1. Vibrio cholera

      2. Escherichia coli

      3. Shigella species

      4. Salmonella species

      5. Campylobacter species

      6. Yersinia enterocolitica

    3. Toxin-mediated

      1. Staphylococcus aureus

      2. Clostridium perfringens

      3. Bacillus cereus

  3. Infectious diarrhea: inflammatory colitis

    1. Shigella species

    2. E coli

    3. Campylobacter species

    4. Salmonella species

    5. Y enterocolitica

    6. Clostridium difficile

    7. Klebsiella oxytoca


The first symptom the patient noted was loss of 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 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?


The pivotal point in this case presentation is the acute onset of watery diarrhea. The early predominance of nausea will also help in making a diagnosis. This presentation certainly speaks for an infectious cause. The low-grade fever and absence of dysentery make it likely that the diagnosis is in the category of gastroenteritis. Table 13-1 lists the differential diagnosis.

Table 13-1.Diagnostic ...

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