Stool Tests for Colorectal Cancer
A stool test is one of many tests used to look for colorectal cancer. Colorectal cancer affects the large intestine (colon) and the rectum. Stool tests look for signs of blood or cancer in a stool sample.
Stool tests include:
- Fecal immunochemical test (FIT).
- Guaiac fecal occult blood test (gFOBT).
- Stool DNA test, also called FIT-DNA.
Blood in the stool may be the only symptom of colorectal cancer. But not all blood in the stool is caused by cancer. Other conditions that can cause blood in the stool include:
- Anal fissures.
- Colon polyps.
- Peptic ulcers.
- Ulcerative colitis.
- Gastroesophageal reflux disease (GERD).
- Crohn's disease.
- Aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs).
A stool test is one of many tests that may be used to screen for colon cancer. Other tests include sigmoidoscopy, colonoscopy, and CT colonography. Which screening test you choose depends on your risk, your preference, and your doctor. Talk to your doctor about what puts you at risk and what test is best for you.
Why It Is Done
Stool tests are done to look for blood in the stool, since cancer and polyps in the colon are more likely to bleed than normal colon tissue. One type of stool test, the FIT-DNA test, checks the stool for blood and genetic changes in DNA that could be signs of cancer. If blood or DNA changes are found, more tests, such as a colonoscopy, will be done to find the cause.
How To Prepare
With the gFOBT, you will be given instructions about foods to avoid in the days before the test. Some medicines may also need to be stopped for a brief time before the test.
Don't do the stool tests during your menstrual period or if you have active bleeding from hemorrhoids. Also, don't test a stool sample that has been in contact with toilet bowl cleaning products that turn the water blue.
How It Is Done
There are different types of home tests. The companies that make the test kits provide instructions. To get accurate results, carefully follow the instructions in your kit.
Fecal immunochemical test (FIT)
The test kit contains the things you need to collect small samples of stool. For some types of FIT, you may need to collect a stool sample on 2 or more days.
The FIT test doesn't require a special diet in the days before you take the test.
When the test is done, follow the instructions to return the test. Some tests provide the results right away. If your test shows that blood was found, call your doctor as soon as possible.
Guaiac fecal occult blood test (gFOBT)
The test kit contains the things you'll need, such as test cards or a special test pad. You may need to collect stool samples over three different bowel movements on three different days. Be sure to follow any instructions about foods or medicines to avoid in the days before the test.
Your instructions may say to:
- Put tiny samples of stool onto paper cards. The cards are either sent to a lab or you will test them at home using a solution from your test kit. A color change means that there is blood in the stool.
- Place a special test pad in the toilet after having a bowel movement. The pad will change color if the stool has blood in it.
If there is blood in your stool, call your doctor as soon as possible.
Combined FIT and stool DNA test (FIT-DNA)
The test kit has a container for you to collect an entire stool sample to send to the lab.
The kit includes instructions that show how to collect a stool sample. You may also be asked to take a small sample from the stool that you collected and seal it into a small plastic tube. The kit contains a bottle of liquid preservative that you will pour into the container to cover the stool before sealing the lid. Follow the instructions for sending the kit to the lab.
There are no known risks from having this test.
If your test sample is sent to a lab or returned to your doctor's office, your test results will likely be read by your doctor. Some labs may send you the results. And depending on the type of test you choose, you may be able to see the results after completing the last step.
A normal FIT or gFOBT test means that there was no blood in your stool at the time of the test. A normal FIT-DNA test means that no blood or changed genes were found. Normal test results are called negative.
An abnormal FIT or gFOBT test means that there was some blood in your stool at the time of the test. An abnormal FIT-DNA test means that some blood or changed genes were found. Abnormal test results are called positive.
Talk with your doctor about how often you should do a test, depending on your age and any risk factors you may have for colorectal cancer.
A benign polyp, a precancerous polyp, or cancer can cause a positive stool test. Abnormal results can happen even if you don't have cancer. But with a positive test, there is a small chance that you could have colorectal cancer.
If your test is positive, you will need to have a colonoscopy. This would be used to see if the stool test result is from colorectal cancer.
But blood in the stool is more often caused by something other than cancer. These other causes could include hemorrhoids, ulcers, or taking aspirin.
Stool test results that are positive when you don't have cancer are called false-positive test results.
Current as of: May 4, 2022
Author: Healthwise Staff
Adam Husney MD - Family Medicine
E. Gregory Thompson MD - Internal Medicine
Kathleen Romito MD - Family Medicine
Arvydas D. Vanagunas MD - Gastroenterology