Colorectal Cancer Screening Tests

Why this test may be ordered

• To check for signs of colorectal cancer before symptoms appear
• To find and remove colorectal polyps that could become cancerous
• Routine screening starting at age 45 for average-risk individuals
• Earlier or more frequent screening if you have risk factors such as family history, inflammatory bowel disease, or obesity

Preparation and collection

Preparation depends on the test type. For colonoscopy and virtual colonoscopy, you need bowel prep: follow a liquid diet for 1-3 days, drink clear liquids, and take a strong laxative or enema. For sigmoidoscopy, you may need a clear liquid diet, fasting, and an enema. For gFOBT, avoid NSAIDs, vitamin C, certain vegetables, and red meat for several days. No special preparation is needed for FIT or stool DNA tests.

Visual exams (colonoscopy, sigmoidoscopy, virtual colonoscopy) are performed in a medical setting using a thin tube with a camera or CT scanner. Stool tests (FIT, gFOBT, stool DNA) are done at home: you collect a stool sample using a kit and mail it to a lab.

There are no risks to having a fecal occult blood test or a stool DNA test. Serious risks of colonoscopy are rare but can include tears in the colon or rectum wall, bleeding, or reaction to the sedative. Sigmoidoscopy has fewer risks, with tears and bleeding even less common. Virtual colonoscopy risks include exposure to a small dose of radiation (this test is not recommended if you are pregnant; tell your provider if you have any implanted medical devices such as a pacemaker) and, rarely, a tear in the colon or rectum when inflated with air. Seek medical care immediately if you have severe abdominal pain, blood in your stool or bleeding from your anus, dizziness, or weakness.

Understanding results

Your results will depend on the type of screening test you had. Below are possible results and what they may mean.

• Screen-negative (normal): No signs of colorectal cancer or polyps were found. For colonoscopy, this means no polyps or abnormal tissue were seen. For stool tests, no hidden blood or genetic changes were detected. Limitation: Stool tests may miss some polyps or cancers. Visual exams may miss small polyps. A normal result does not guarantee the absence of all abnormalities. Next step: Continue routine screening as recommended by your provider. For colonoscopy, if no polyps were found, repeat every 10 years. For sigmoidoscopy, repeat every 5 years. For virtual colonoscopy, repeat every 5 years. For FIT or gFOBT, repeat every year. For stool DNA, repeat every 3 years.
• Screen-positive (abnormal): Signs of possible polyps, abnormal tissue, or blood were found. This does not necessarily mean you have cancer. Limitation: Stool tests can have false positives (show blood when there is none) and cannot detect precancerous changes. Visual exams may find polyps that are benign. An abnormal result does not necessarily mean you have cancer. Next step: Your provider will likely order more tests, such as a colonoscopy, to help make a diagnosis. If polyps were found and removed during colonoscopy, they will be sent to a lab for testing. Depending on the size and number of polyps, your provider may recommend more frequent colonoscopies.

• Stool tests can have false positives and may miss some polyps or cancers.
• Stool tests cannot detect precancerous changes.
• Visual exams may miss small polyps.
• Virtual colonoscopy cannot remove polyps; a traditional colonoscopy is needed if abnormal.
• Sigmoidoscopy only looks at the lower third of the colon.

What to do next

Discuss your results with your healthcare provider to determine the best follow-up plan. If your test was abnormal, you may need a colonoscopy for further evaluation. If your test was normal, continue with routine screening as recommended.

• What type of colorectal cancer screening test is right for me?
• How often should I get screened?
• What are the benefits and risks of each screening option?
• Do I need to prepare for the test, and if so, how?
• What do my results mean and what are the next steps?

Evidence source

U.S. National Library of Medicine: Colorectal Cancer Screening Tests