How Diarrhea Is Diagnosed

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While you may be able to 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 doctor may conduct various tests to determine the diagnosis. 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 doctor's office or at an urgent care clinic.

Self-Checks/At-Home Testing

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 doctor if diarrhea clears up in 24 hours for children or 48 hours for adults.

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

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

Make notes of any food, travel, untreated water, and medications you were taking before you developed diarrhea. 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 doctor want a stool sample to test. A small amount of the 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 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.

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.

Clostridium Difficile Toxin

Clostridium 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

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

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 by the presences of red food such as Jello or beets.

Fecal Fat

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 Antibody 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 diagnosis of 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. These procedures usually require sedation and are done at a hospital or imaging clinic, often by a specialist called a gastroenterologist. Your doctor will give you the preparation steps needed beforehand, and you will need a responsible person to transport you home from the procedure.

Ultrasound and CT scans of the abdomen are needed in some cases of chronic diarrhea to look at the bowel, pancreas, and other organs. These procedures have replaced using an upper GI X-ray series or a barium enema.

Differential Diagnoses

In the case of acute diarrhea, your doctor 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 lactose intolerance or fructose intolerance), 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 will lead your doctor 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 doctor.

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