Testing for Diarrhea

By Krista Williams, BSc, DVM; Kristiina Ruotsalo, DVM, DVSc, Dip ACVP & Margo S. Tant BSc, DVM, DVSc

What causes diarrhea in a pet?puppy_garbage

There are many different diseases and disorders that can cause diarrhea in pets. It can be as simple as a pet having intestinal parasites (worms) or eating spoiled food. However, in many cases the cause is more difficult to determine.

The following are just a few of the many causes of diarrhea:

  • Disorders of stomach and bowel (gastrointestinal tract) including inflammation; bacterial, viral, or fungal infections, parasites; tumors; inability to digest and absorb nutrients; changes in diet; food allergies; and eating inappropriate things (dietary indiscretion).
  • Hormonal diseases such as hypoadrenocorticism (failure of the adrenal glands called Addison's disease) and hyperthyroidism in cats (over active thyroid gland).
  • Systemic diseases (which affect the whole body) such as kidney failure, liver disease, and pancreatitis.
  • Certain drugs and toxins.


This list is huge! How can we possibly determine the cause in my pet?

The search for answers starts with a complete history and physical examination. A pet’s history is the information you give your veterinarian about your pet’s illness. This would include details about the frequency, urgency, and appearance of the diarrhea. Other useful information would include whether the pet has eaten anything unusual such as house plants or table scraps, or whether the pet is showing other signs of illness such as weight loss, poor appetite, or vomiting. To help you put this history together before visiting your veterinarian, please see the handout "Diarrhea Questionnaire and Checklist".

Physical examination involves looking at all parts of the body, and includes listening to the heart and lungs with a stethoscope, and palpating the abdomen (gently squeezing or prodding the abdomen with the fingertips to assess the internal organs). Physical examination may give clues about the cause of diarrhea. For example, a lethargic puppy with a fever and bloody diarrhea may have parvovirus infection; a cat with thickened bowels may have intestinal cancer; and a dog in good body condition that is passing small amounts of diarrhea frequently may have colitis (inflammation of the lower bowel).

"Sometimes a diagnosis can be made based on history and physical examination, but in most cases additional diagnostic tests will be needed."

Sometimes a diagnosis can be made based on history and physical examination, but in most cases additional diagnostic tests will be needed. Your veterinarian may recommend doing screening tests which are basic tests that provide information about the overall health of your pet and may provide further clues about the underlying problem.

What screening tests are recommended and why?

In a pet with diarrhea the most commonly recommended screening tests include: complete blood count (CBC), serum biochemistry profile, urinalysis, and fecal parasite tests. In middle aged to older cats, a serum thyroxine (total T4) concentration is also recommended. Screening tests may provide clues to the cause of the diarrhea. In addition, they may uncover problems caused by diarrhea such as electrolyte disturbances and dehydration.

Complete Blood Count. This is a simple blood test that provides information about the different cell types in blood. These include red blood cells, which carry oxygen to the tissues, white blood cells, which fight infection and respond to inflammation, and platelets, which help the blood to clot. The CBC provides details about the number, size, and shape of the various cells types, and identifies the presence of abnormal cells in circulation (see handout “Complete Blood Count”).

In a pet with diarrhea, some changes that could be seen on a CBC include:

  • Anemia (a decrease in the number of red blood cells, packed cell volume, and hemoglobin). This could be a sign of bleeding in the bowel or a longstanding disorder such as Addison's disease, liver disease, or cancer.
  • Hemoconcentration (an increase in the number of red blood cells, packed cell volume, and hemoglobin). This indicates dehydration.
  • High numbers of white blood cells. This often indicates underlying infection or inflammation and may be seen with pancreatitis, liver disease, kidney infection, and some cancers.
  • Low numbers of white blood cells could indicate viral infection such as distemper or parvovirus in dogs and panleukopenia in cats.
  • Increased numbers of eosinophils (a type of white blood cell) could indicate allergies or parasitic infections, both of which can cause diarrhea.

Serum biochemistry profile refers to the chemical analysis of serum (the liquid part of blood). There are many substances in serum, including proteins, enzymes, fats, sugars, hormones, and electrolytes. Measuring the levels of the various substances in the blood provides information about the health of the body’s organs and tissues such as the liver, kidney, and pancreas. Changes and abnormalities found in the biochemistry profile can help to diagnose a variety of diseases and disorders (see handout “Serum Biochemistry”).

In a pet with diarrhea, some changes that could be seen on a serum biochemistry profile include:

  • Abnormally high levels of liver enzymes. These changes are associated with liver damage or dysfunction and could explain a pet’s diarrhea.
  • Increased kidney values - BUN (blood urea nitrogen) and creatinine. Very high levels of these two substances suggest kidney failure, which may contribute to diarrhea.
  • Low albumin (blood protein). This suggests a pet may be losing excessive amounts of protein in the diarrhea.
  • Elevated levels of lipase. Lipase is an enzyme that comes from the pancreas. Increased levels are sometimes seen in pets with pancreatitis.
  • Pancreatic specific lipase (PSL), spec fPL, or spec cPL may be included in the biochemistry profile to more accurately assess the health of the pancreas.
  • Altered electrolytes. Electrolytes are salts and minerals found in the blood. Changes in electrolytes, especially sodium and potassium, are often seen with kidney disease and Addison's disease, and can also be an indicator of electrolyte disturbance caused by diarrhea.
"Urinalysis is important for the accurate interpretation of changes noted on the serum biochemistry profile."

Urinalysis is a simple test that analyses the physical and chemical composition of urine. It measures how well the kidneys are working, identifies inflammation and infection in the urinary system, and helps to detect diabetes and other metabolic disturbances. Urinalysis is important for the proper interpretation of the serum biochemistry profile and should be done at the same time as blood testing (see handout “Urinalysis”).

In a pet with diarrhea some changes that could be seen on a urinalysis include:

  • Low specific gravity. This indicates the pet is passing dilute (pale watery urine) urine. If serum BUN and creatinine are elevated at the same time, then kidney disease or possibly kidney failure is present and could be contributing to diarrhea.
  • High specific gravity. This indicates the pet is passing highly concentrated urine (dark yellow urine). It may be a sign of dehydration caused by the loss of fluid in the diarrhea.
  • Bilirubin. The presence of bilirubin pigment in the urine, especially in the cat, is a sign of liver disease, which may be contributing to the diarrhea.

Intestinal parasite tests. There are many intestinal parasites that can cause diarrhea especially in puppies and kittens. There is no one test that can detect all intestinal parasites and several different tests may be used. The simplest test is fecal flotation in which a small amount of fresh stool is prepared and examined microscopically for the presence of parasites or parasite eggs. Multiple fecal flotations may be needed to detect infection. A simple version of the fecal flotation can be done in the veterinary clinic or a sample can be sent to the laboratory for a slightly better test that uses a different technique

Some intestinal parasites such as Cryptosporidium, Toxoplasma, Giardia, and Tritrichomonas are difficult to detect with flotation methods. Specialized testing including blood tests may need to be done. Samples may be sent to a veterinary referral laboratory for these tests.

Serum thyroxine (total T4). This test is used to diagnosis hyperthyroidism in cats. Hyperthyroidism is caused by an overactive thyroid gland and is common in middle-aged to older cats. The gland produces excessive amounts of thyroid hormones, which substantially increases the body’s metabolic rate and often causes vomiting and diarrhea.

"Affected cats typically have markedly elevated levels of T4 in their blood."

Most cases can be diagnosed with a single blood test to measure the level of total thyroxine (T4) in the blood stream. Affected cats typically have markedly elevated levels of T4 in their blood.

Do all of these screening tests need to be performed if my pet is feeling well other than a bit of mild diarrhea?

No. If your pet is bright and alert and the physical examination is normal, then screening tests may not be recommended right away. This is especially true if there is an obvious reason for the diarrhea, such a sudden change in diet or a history of eating table scraps. However, if diarrhea continues or reoccurs, or your pet shows other signs of illness such as fever or abdominal pain, then screening tests are strongly recommended.

What additional tests might be recommended for a pet with diarrhea?

There are many additional tests that might be recommended depending on the results of a pet’s history, physical examination, and screening tests. If the diarrhea seems to be caused by problems that do not involve the gastrointestinal tract, such as liver disease, kidney failure, Addison’s disease, or heart disease for example, then specific testing for those conditions would be recommended.

"There are many additional tests that might be recommended depending on the pet’s history, physical examination, and screening tests."

If screening tests suggest the problem involves only the gastrointestinal tract then further testing would focus on disorders of the stomach and bowel such as problems with the pancreas; bacterial, fungal, or viral bowel infection; inflammatory bowel disease; neoplasia (tumor); food hypersensitivity; and so on. Some examples of testing that might be recommended include:

  • Fecal culture. Special techniques are used to culture stool material to detect specific bacteria known to cause diarrhea.
  • Fecal antigen. Special techniques are used to detect parasites such as Giardia and whipworm that are sometimes difficult to detect with other methods.
  • Fecal cytology (direct fecal smear evaluation). A veterinary pathologist examines a thin smear of fecal material under the microscope to look for specific bacteria called Clostridia, other organisms that cause bacteria, as well as inflammatory cells, and neoplastic cells.
  • Fecal DNA testing. More recent tests using DNA technology are helpful to detect viral, bacterial, and parasitic infections that can cause diarrhea.
  • Imaging studies including X-rays or ultrasound could be used to examine abdominal organs and the digestive system.
  • Endoscopy or exploratory surgery are used to look at the bowel directly to find abnormalities.
  • Trypsin-like immunoreactivity (TLI). This test is used to detect exocrine pancreatic insufficiency, a disorder in which food is not digested properly resulting in diarrhea.
  • Biopsy. Fine needle biopsies and tissue biopsies may be collected during ultrasound, endoscopy, or exploratory surgery and may provide a definitive diagnosis.
  • Food trials. Food hypersensitivity (like an allergy) is a common cause of diarrhea; feeding special foods may help to diagnose food hypersensitivity.
Related Articles