How Diarrhea Is Diagnosed

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While you may be able to safely wait out a short bout of diarrhea, it can be a serious problem, especially in children, infants, and the elderly. Some causes of diarrhea require treatment, and your healthcare provider may conduct various tests to determine the diagnosis. Chief among them are stool tests and blood tests.

Diarrhea can result in dehydration, pain, weakness, and malnutrition if untreated. If you experience sudden diarrhea that lasts for longer than around 48 hours, you should seek medical treatment at your practitioner's office or at an urgent care clinic.

Doctor looking at sample under microscope
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Diarrhea is loose, watery stools occurring three or more times in one day. Depending on the cause, you may have other symptoms.

Typically, you won't need to see your healthcare provider if diarrhea clears up in 24 hours for children or 48 hours for adults. Take note, however, of other symptoms that may indicate a more concerning case.

You should see your healthcare provider if you note any of these signs:

  • Stool that is black or tarry, or contains blood or pus
  • 102-degree or higher fever
  • Severe abdominal or rectal pain in adults
  • Chronic diarrhea or diarrhea lasting for more than two days
  • Signs of dehydration such as dizziness, headache, dark urine, no urine, dry mouth, nose, or tongue

Diarrhea in a newborn or infant should always been evaluated by a pediatrician.

It is also useful to write down any food you ate, trips you took, untreated water you drank, and medications you were taking before you developed diarrhea. As much detail as possible can be useful to your healthcare provider.

As there have been many outbreaks of illness from contaminated food products, save any containers or wrappers in case authorities want to track the sources.

Labs and Tests

In addition to a physical examination and taking your medical history, your practitioner want a stool sample to test. A small amount of stool should be collected in a sterile container to prevent contamination. This can be done at your primary care provider's office, urgent care, or at an emergency room if the condition has become severe.

Blood and urine tests may also be done to check for signs of infection, anemia, dehydration, and electrolyte imbalance.

Stool Tests

There are several tests that may be conducted on the stool sample that you provide.

Stool Culture

This test examines stool for organisms that should not be present, or are present in too high of numbers, including Campylobacter, Salmonella, and Shigella. The culture will determine what organisms are present and if they could potentially be causing the problem.

Stool Panel for Gastrointestinal Pathogens

This panel looks for the genetic material of specific pathogens (viruses, bacteria, and parasites) that commonly cause diarrhea. It is much faster than older methods, such as stool culture, and the results may be available within a few hours rather than days.

Clostridium Difficile Toxin Test

Clostridioides difficile, also known as C. difficile or C. diff, is a bacteria that causes chronic diarrhea. This bacteria, and others like it, may be present normally in the gut but begin to grow in numbers after you have been treated with an antibiotic. This overgrowth can cause chronic diarrhea.

Ova and Parasites Test

This test looks for signs of a parasite living in the intestinal tract. A medical technologist examines the stool sample under a microscope for parasites and their eggs.

Swimming in or drinking untreated water (such as while camping or hiking) can expose you to Giardia lamblia and organisms. Travel to foreign countries and eating undercooked meat can also expose you to these organisms.

Fecal Occult Blood Test

This test looks at the stool to see if there is blood present that cannot be seen with the naked eye. It can also determine if the presence of a red color in the stool is caused by blood or the consumption of naturally red or red-dyed foods, such as beets or gelatin.

Fecal Fat Test

This test looks at the fat level in stool. Fat in the stool may indicate a problem with the intestines and the ability to absorb nutrients.

Stool Antigen Tests

These tests check for antigens in the stool that may indicate the presence of rotavirus or parasites such as GiardiaCryptosporidium, and Entamoeba histolytica.

Fecal Elastase

​This test looks at how well the pancreas is performing its digestive function of secreting enzymes that help the body digest food. If the pancreas is not performing well, food will be incompletely digested, leading to chronic diarrhea that can be severe or worsen over time. This condition is called exocrine pancreatic insufficiency (EPI).

Fecal Trypsin/Chymotrypsin

This stool test looks for enzymes that should be in the stool if the pancreas is working normally. Low levels may point to a pancreatic disease or cystic fibrosis.

Blood Tests

Specific blood antibody tests may be ordered to help clarify diagnoses. These can include antibodies for specific parasites, celiac disease antibodies, and yeast antibodies.


In most cases, the cause of diarrhea can be determined without imaging procedures. But in some cases, the diagnosis may need imaging using endoscopy, colonoscopy, or sigmoidoscopy.

In these procedures, a tube with an imaging device is inserted into the gastrointestinal tract. They usually require sedation and are done at a hospital or imaging clinic, often by a specialist called a gastroenterologist. Your healthcare provider will give you the preparation steps needed beforehand, and you will need someone to transport you home from the procedure.

Differential Diagnoses

In the case of acute diarrhea, your practitioner will look at the wide variety of possible causes, including food poisoning, bacteria, viruses, parasites, and medications. Some of these causes will require treatment with antibiotics or anti-parasitic medications. Other causes will be self-limiting, but you may need supportive measures to prevent dehydration and other complications.

Chronic diarrhea can take more time to diagnose. It can be due to food allergies or intolerances (such as to lactose or fructose), celiac disease (an autoimmune reaction to gluten), irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), malabsorption syndromes, chronic pancreatitis, and more.

Classifying chronic diarrhea as fatty, inflammatory, or watery can lead your healthcare provider on the diagnostic path. It is rare that chronic diarrhea is due to colon cancer, polyps, or ischemia of the gut.

A Word From Verywell

Diarrhea is a sign that something has upset your digestive system. While it will usually go away after a day or two, it can lead to serious complications like dehydration or malnutrition if it persists. If you have severe symptoms or chronic diarrhea, see your healthcare provider.

Frequently Asked Questions

  • What tests are done to diagnose diarrhea?

    Stool testing is commonly used to diagnose diarrhea. Tests can include stool culture, a stool panel for gastrointestinal pathogens, Clostridioides difficile toxin test, ova and parasite tests, fecal occult blood test, fecal fat test, stool antigen tests, fecal elastase, and fecal trypsin/chymotrypsin. Blood tests are also sometimes used to test for antibodies to specific parasites, yeast, or gluten.

  • How do I collect a stool sample for diarrhea?

    When stools are loose, it can be difficult to get a sample. The neatest way to do this is to use what is known as a nurse's hat, which is commonly used for collecting urine samples. If you don’t have access to a nurse's hat, you can use a disposable bowl and a plastic bag or plastic wrap. Attach the bag or sheet under the toilet seat and put a bowl inside it. Once you’ve collected the sample, you will need to transfer it to the specimen collection jar. 

  • What is a diarrhea panel?

    A stool panel for gastrointestinal pathogens looks for the genetic material of bacteria, parasites, and viruses that commonly cause diarrhea. 

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. U.S. National Library of Medicine, Medline Plus, "Diarrhea"

  2. American College of Gastroenterology, "Diarrheal Diseases—Acute and Chronic"

  3. American Association for Clinical Chemistry Lab Tests Online, "Diarrhea"

By Jennifer Whitlock, RN, MSN, FN
Jennifer Whitlock, RN, MSN, FNP-C, is a board-certified family nurse practitioner. She has experience in primary care and hospital medicine.