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Before any diagnostic testing is performed, a doctor will want to learn about a person's diarrhea. The frequency, quantity, and consistency of normal bowel movements are very individual. They depend on a person's metabolism, the foods they eat, their activity level, the amount of fluids they drink, the medications they take, and even any stress that they are experiencing. A doctor will be looking for things that are out of the ordinary and for alterations that are commonly associated with diarrhea-causing conditions. A doctor may ask a series of questions that allow him to make informed choices about which laboratory tests will be the most appropriate, including:

  • What is the frequency of your stools?
  • How long you have been experiencing diarrhea?
  • What are the consistency, color, and approximate volume of the stools?
  • Is there blood or mucus in the stool?
  • What other symptoms are you having: abdominal pain, nausea, fever, headache, fatigue?
  • What and where have you eaten recently?
  • Have you been camping? Have you traveled outside of the U.S.? Where?
  • Are any of your family members, close acquaintances, or co-workers ill?
  • Have you been on antibiotics lately?

If a person's diarrhea is uncomplicated and goes away within a few days, the doctor may not search for the cause of the gastrointestinal upset. He may order one or more tests, however, if the diarrhea is severe, if there is blood or mucus present, or if it is continuing unabated. This is especially true if the person has been outside the United States and/or has eaten or drunk anything that has also made someone close to them ill.

Laboratory tests

Testing may include tests to identify diarrhea caused by a pathogen, such as:

  • O&P (Ova and Parasite) testing. A microscopic evaluation of the stool for parasites and the ova (eggs, cysts) of parasites.
  • Stool WBC. White blood cells may be present in the stool when there is a bacterial infection.
  • Stool Culture. To check for pathogenic gastrointestinal bacteria.
  • Antigen tests for giardia, cryptosporidium and E. histolytica. These tests detect protein structures on the parasites. They are more sensitive and specific for these particular parasites than the O&P microscopic exam.
  • Antibody tests for parasites. These are not as useful to detect current infections but may be ordered to check for past or chronic infections, especially if unusual parasitic infections are suspected.
  • Rotavirus test. A rapid antigen test.
  • Clostridium difficile toxin. A rapid test used to identify a Clostridium difficile infection, by detecting the toxin made by the bacteria.

Tests for non-infectious causes of diarrhea include:

Sometimes a biopsy of the small intestines may be performed to look for signs of parasitic infection or celiac disease.

Non-laboratory tests 
Sometimes, special procedures are used to look at portions of the gastrointestinal tract. See the National Institute of Diabetes and Digestive and Kidney Diseases web site pages on:

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