Fecal Occult Blood

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

What is fecal occult blood?

Fecal occult blood refers to the presence of small quantities of blood in the stool that cannot be seen with the naked eye (occult means hidden from view). The blood can come from anywhere in the digestive tract, including the mouth, stomach, small intestine, large intestine, or rectum.

Why test for fecal occult blood?

Just as a water tap left dripping can waste a lot of water, even a small amount of bleeding into the digestive tract may result in substantial blood loss if it is not detected quickly. Left untreated, bleeding can lead to anemia or iron-poor blood. The fecal occult blood test is a simple, non-invasive way to determine if there is bleeding in the digestive system.

What causes blood in stool?

Blood can be found in the stool for many reason, but some common causes include severe dental disease, internal parasites, stomach or intestinal ulcers, bowel inflammation, tumors, or foreign bodies the pet has swallowed, such as small toys and other indigestible material. If the fecal occult blood test is positive, your veterinarian may recommend further testing to determine where the bleeding is occurring.

What sample is required?

A small amount of fresh stool is all that is needed. Your veterinarian or veterinary technician may collect stool material directly from your pet's rectum using a gloved finger, or you may be asked to provide a sample of fresh stool from the pet. Ideally, the stool sample should be less than 24 hours old.

How does the test work?

Blood contains a substance called hemoglobin. When a stool sample containing blood is placed on a special testing material, the chemical activity of hemoglobin causes a visible color change in the testing material. This color change signals the presence of blood.

Does the test always work?

Although the fecal occult blood test is simple and easy, it may give incorrect results in certain situations.

For example, the test may fail if blood is not distributed evenly throughout the stool sample, or if bleeding occurs intermittently rather than continuously. In these situations, the amount of blood in the stool will be quite variable and as a result a particular stool sample could show negative for occult blood, even though there is bleeding from the digestive system.

"Diet can affect the accuracy of the fecal occult blood test."

Diet can also affect the accuracy of the fecal occult blood test. If a pet eats raw or undercooked meat, raw vegetables, or even some types of canned food, the test for fecal occult blood may be positive, even though the pet is not bleeding into the digestive tract. To avoid this problem, your veterinarian may suggest feeding dry kibble exclusively for three days prior to the test, and recommend that meat, meat scraps, and raw vegetables not be fed to your pet during this interval.

Because of variability in the success of the fecal occult blood test, your veterinarian may suggest repeating the test to ensure an accurate diagnosis.

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