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Before any diagnostic testing is performed, a healthcare practitioner 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 healthcare practitioner will be looking for things that are out of the ordinary and for alterations that are commonly associated with diarrhea-causing conditions. A healthcare practitioner may ask a series of questions to help 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 healthcare provider may not search for the cause of the gastrointestinal (GI) upset. The healthcare practitioner 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

Examples of Tests for Infectious Causes of Diarrhea

Test Sample Description
Panel of tests for gastrointestinal pathogens Stool Molecular tests that detects specific viruses, bacteria and parasites; the pathogens detected depends on the panel of tests that is used.
Stool culture Stool Cultures of stool samples use special nutrient media that selectively allow pathogens to grow while inhibiting growth of bacteria that are normally present in the digestive tract (normal flora). They may detect several different bacteria, but laboratories typically evaluate stool cultures for the most common intestinal pathogens:
  • Campylobacter species
  • Salmonella species
  • Shigella species
Clostridium difficile toxin test Stool Detects toxin produced by C. difficile
Tests specific for Shiga toxin-producing Escherichia coli (STEC) Stool
  • Toxin test—detects the Shiga toxin directly (used in conjunction with stool cultures)
  • Pulsed-field Gel Electrophoresis (PFGE)—used by public health laboratories to identify subtypes of E. coli that are suspected in an outbreak
  • Genetic tests—rapid molecular tests (PCR) for Shiga toxin 1 gene (stx1) and Shiga toxin 2 gene (stx2)
Stool WBC Stool Detects white blood cells that may be present in the stool when there is a bacterial infection, such as Clostridium difficile
Antigen test for rotavirus Stool A rapid antigen test that can detect a common cause of diarrhea in children
Ova and parasite exam Stool A microscopic evaluation of the stool for parasites and the ova (eggs, cysts) of parasites.
Antigen tests specific for the parasites:
  • Giardia lamblia
  • Entamoeba histolytica
  • Cryptosporidium parvum
Stool Detects protein structures on the parasites; they are more sensitive and specific for these particular parasites than the O&P microscopic exam.
Antibody tests for GI parasites Blood Some reference laboratories may offer these tests. They are not as useful to detect current infections but may be ordered to check for past or chronic infections.

Examples of Tests for Non-infectious Causes of Diarrhea

Test Sample Description
Trypsin/Chymotrypsin Stool Trypsin and chymotrypsin are enzymes that are detectable in the stool if the pancreas is functioning normally. These tests are used to help evaluate pancreatic function. Low levels are seen with pancreatic diseases and cystic fibrosis (CF).
Stool WBC Stool Increased number of white blood cells in the stool may indicate inflammatory bowel disease (IBD).
Fecal fat Stool Measures fat level in the stool; increased level is associated with malabsorption that can be caused by celiac disease, pancreatic insufficiency, or cystic fibrosis, for example.
Food allergy tests Not applicable Oral food challenges are considered the "gold standard" for diagnosing food allergies. They are labor-intensive and require close medical supervision because reactions can be severe, including life-threatening anaphylaxis. Food elimination is another way to test for food allergies: eliminating all suspected foods from the diet, then reintroducing them one at a time to find out which one(s) are causing the problem.
Celiac disease antibody tests Blood Tissue transglutaminase antibody (tTG), IgA class is the primary test ordered to screen for celiac disease. It is the most sensitive and specific blood test for celiac disease. Other tests may include the IgG class of tTG, total IgA and deamidated gliadin peptide antibodies (anti-DGP), IgA or IgG.
Lactose tolerance testing Breath, blood Measures hydrogen in the breath or changes in the level of glucose in the blood after a person is given a drink containing a standard amount of lactose, thus determining whether the individual is capable of proper digestion of lactose
Xylose absorption test Blood, urine Determines how well someone absorbs xylose; it measures the level of xylose in the blood and urine after a standard amount is ingested in order to evaluate the person's ability to absorb carbohydrates in general.
Lactoferrin and calprotectin tests Stool Detect inflammation associated with IBD
Anti-Saccharomyces cerevisiae mannan antibodies (ASCA) Blood Detects anti-Saccharomyces cerevisiae antibodies (ASCA), which are immune proteins frequently present in people who have IBD
Fecal occult blood test and fecal immunochemical test (FOBT, FIT) Stool Detects digestive tract bleeding that may indicate colon cancer
5-HIAA, Chromogranin A, Serotonin Blood These are tests to help diagnose a carcinoid tumor, which is a relatively rare cause of chronic diarrhea.

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

Some tests may be done to help check the general health of the individual and/or to help give clues to the cause of the diarrhea:

  • Complete Blood Count (CBC)—may be used to check for anemia, for example, or to help detect an infection. For example, a type of white blood cell called an eosinophil may be elevated with parasitic infections.
  • Electrolytes—if a person is dehydrated, a healthcare practitioner may want to do a blood test to determine whether the person's electrolytes have become imbalanced.
  • Osmolality—blood, urine, or stool samples may be examined to evaluate electrolyte balance.

Non-laboratory tests
If the cause of diarrhea is not determined through a physical exam, medical history, or initial testing, special imaging procedures may be 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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