Colorectal Cancer | Diagnosis & Tests


What are screening tests for colorectal cancer?

Screening tests can find polyps or cancers before they are large enough to cause any symptoms. Screening tests are important because early detection means that the cancer can be more effectively treated. Your doctor will choose the tests that are right for you. The following are some screening tests for colorectal cancer:

Digital Rectal Exam. In this exam, your doctor puts his or her gloved finger into your rectum to find any growths. This exam is simple to do and is not painful. However, because this exam can find less than 10% of colorectal cancers, it must be used along with another screening test.

Barium Enema. For this test, you are given an enema (injection of fluid into the rectum) with a liquid that makes your colon show up on an X-ray. Your doctor looks at the X-ray to find abnormal spots in your entire colon. If you have an abnormal spot or if the radiologist detects polyps in your colon, your doctor will probably want you to have colonoscopy.

Fecal Occult Blood Test. This test checks your stool for blood that you can't see. Your doctor gives you a test kit and instructions to use it at home. Then you return a stool sample to your doctor for testing. If blood is found, another test is done to look for a polyp, cancer or another cause of bleeding. Your doctor will also ask you to not eat certain foods or take certain medicines that may interfere with test results a few days before the test.

Certain foods and medicines can make this test turn out positive, even though you don't really have blood in your stool. This is called a "false-positive" test. These include some raw vegetables, horseradish, red meat, non-steroidal anti-inflammatory drugs (such as ibuprofen), blood thinners, vitamin C supplements, iron supplements and aspirin. Some medical conditions, like hemorrhoids, can also cause a false-positive test result.

Stool DNA Test. This test checks your stool for cells that are shed by colon cancers or precancerous polyps. Your doctor will give you a test kit with instructions on how to collect a stool sample. Your doctor may also ask you to not eat certain foods or take certain medicines that may interfere with test results a few days before the test. If your test turns out positive, your doctor will probably want you to have a screening test called colonoscopy.

Colonoscopy. Before you have this test, you are given a medicine to make you relaxed and sleepy. A thin, flexible tube connected to a video camera is put into your rectum, which allows your doctor to look at your entire colon. The tube can also be used to remove polyps and cancers during the exam. Colonoscopy may be uncomfortable, but it is usually not painful.

Virtual Colonoscopy. This is a new test that uses a computerized tomography (CT) machine to take pictures of your colon. Your doctor can then see all of the images combined in a computer to check for polyps or cancer. If your doctor finds polyps or other abnormalities in your colon, you will need to have a traditional colonoscopy to examine them in more detail or to remove them.

Flexible Sigmoidoscopy. In this test, your doctor puts a thin, flexible, hollow tube with a light on the end into your rectum. The tube is connected to a tiny video camera so the doctor can look at the rectum and the lower part of your colon.

This test can be a bit uncomfortable, but it lets your doctor see polyps when they are very small (before they can be found with a fecal occult blood test). Because flexible sigmoidoscopy may miss cancerous polyps that are in the upper part of the colon, some doctors prefer a colonoscopy. Your doctor will discuss these options with you.


Update on Colorectal Cancer by DR Rudy, MJ Zdon (American Family Physician March 15, 2000,

Written by editorial staff

Reviewed/Updated: 04/14
Created: 09/00