About Testing

A colonoscopy allows the doctor to look inside the entire rectum and colon while a patient is sedated. A flexible, lighted tube called a colonoscope is inserted into the rectum and the entire colon to look for polyps or cancer. During this procedure, a doctor can remove polyps or other tissue for examination (see “Biopsy” in the Diagnosis section). The removal of polyps can also prevent colorectal cancer.

Computed tomography (CT or CAT) colonography

CT colonography, sometimes called virtual colonoscopy, is a screening method being studied in some centers. It requires interpretation by a skilled radiologist to provide the best results. A radiologist is a doctor who specializes in obtaining and interpreting medical images. However, CT colonography may be an alternative for people who cannot have a standard colonoscopy due to the risk of anesthesia, which is medication to block the awareness of pain, or if a person has a blockage in the colon that prevents a full examination.

Sigmoidoscopy

A sigmoidoscopy uses a flexible, lighted tube that is inserted into the rectum and lower colon to check for polyps, cancer, and other abnormalities. During this procedure, a doctor can remove polyps or other tissue for later examination. The doctor cannot check the upper part of the colon, the ascending and transverse colon, with this test. This screening test allows for the removal of polyps, which can also prevent colorectal cancer, but if polyps or cancer are found using this test, a colonoscopy to view the entire colon is recommended.

Fecal occult blood test (FOBT) and fecal immunochemical test (FIT)

A fecal occult blood test is used to find blood in the feces, or stool, which can be a sign of polyps or cancer. A positive test, meaning that blood is found in the feces, can be from causes other than a colon polyp or cancer, including bleeding in the stomach or upper GI tract and even eating rare meat or other foods. There are 2 types of tests: guaiac (FOBT) and immunochemical (FIT). Polyps and cancers do not bleed continually, so FOBT must be done on several stool samples each year and should be repeated every year. Even then, this screening test provides a fairly small reduction in deaths from colorectal cancer, around 30% if done yearly and 18% if done every other year.

Double contrast barium enema (DCBE)

For patients who cannot have a colonoscopy, an enema containing barium is given, which helps make the colon and rectum stand out on x-rays. A series of x-rays is then taken of the colon and rectum. In general, most doctors would recommend other screening tests because a barium enema is less likely to detect precancerous polyps than a colonoscopy, sigmoidoscopy, or CT colonography.

Stool DNA tests

This test analyzes the DNA from a person’s stool sample to look for cancer. It uses changes in the DNA that occur in polyps and cancers to find out if a colonoscopy should be done.