What to know about tests for colon cancer

Colorectal cancer includes cancer of the colon (the large intestine) or rectum (the last several inches of the large intestine). More than 150,000 new cases are diagnosed each year, according to the American Cancer Society (ACS), and more than 50,000 people die of the disease. It is the second leading cause of cancer-related deaths in the United States.

Despite its prevalence and deadliness, about half of colorectal cancer deaths could be prevented with early screening, according to the ACS. Screening tests for colorectal cancer can detect polyps or tumors earlier, when treatment is often more successful. (Polyps are precancerous growths that may become cancer over time. Tumors are solid masses of cancer cells.) Here's what to know about screening tests for colorectal cancer.

When to start screenings

The ACS and the United States Preventive Services Task Force (USPSTF) recommend starting screening for colon cancer at age 45. If you are at increased or high risk of colorectal cancer, you should get screened earlier. Risk factors may include having had colorectal cancer or polyps in the past, a history of inflammatory bowel disease (chronic inflammation of the digestive tract), or a family history of colorectal cancer. Discuss these and other risk factors with your healthcare provider (HCP) to help decide when to start screening.

Also discuss lifestyle changes you can make to help lower your risk of colorectal cancer. These include managing your diet, getting as much physical activity as you are able, and drinking alcohol in moderation if you drink. If you smoke, talk to HCP about quitting, and how your health insurance may help cover costs.

Preventive screening options

The colonoscopy is considered the gold standard (preferred method) for detecting colorectal cancer, but there are several other options worth discussing with your HCP. Here’s what you should know about each one:

Colonoscopy

Frequency: Once every 10 years

Preparation: As with most colon-screening tests, your HCP needs a clear view of your colon. That means it needs to be clean and empty. Your HCP will provide you with detailed instructions on how to prepare.

One of the most common ways involves drinking 2 to 4 quarts of a liquid laxative (a substance that causes diarrhea) the night before and the morning of the procedure. You’ll also be asked to stop eating solid food and to drink only clear liquids for at least one day before the test. If you take any medications, talk with your HCP ahead of time about how to manage them before the procedure.

About the test: A colonoscopy is a routine procedure that can be done at an HCP’s office, a clinic, or a hospital outpatient department. It usually lasts from 30 minutes to an hour. To minimize discomfort, you’ll first receive a sedative to make you sleepy. To begin, your HCP inserts a colonoscope—a thin tube with a small video camera on the end—through your rectum and into your colon, providing a full view of all five feet of your colon.

One advantage of this type of screening is that if polyps are found, your HCP can remove them during the colonoscopy. If a polyp is removed, it’s sent to a laboratory to check for signs of cancer. If your HCP sees any irregularity, such as a tumor or larger polyp, they will likely take a tissue sample (biopsy) for further examination.

After the test: Because the sedative will make you groggy, you’ll be asked to arrange for someone to accompany you home. You may also feel bloated, gassy, or have cramps for the rest of the day.

CT colonography (virtual colonoscopy)

Frequency: Once every five years

Preparation: Much like a colonoscopy, you’ll need to empty your colon by drinking a liquid laxative solution the night before and the morning of the screening. You’ll also be asked to eat a clear liquid diet a day or two before the procedure.

About the test: The “virtual” colonoscopy only takes about 10 minutes. It doesn’t require sedation and is done in a facility with a CT scanner. (This device uses X-rays and a computer to take a detailed picture of the inside of your body.)

To begin, you’ll be positioned on the scanner table so that a small, flexible tube can be placed into the rectum. Air is then pumped inside the tube to inflate the colon, offering a clearer view of the area. The table then slides into the scanning machine and a special computer program takes images of the colon, generating two-dimensional X-rays along with a three-dimensional view. The first scan is taken while you lie on your back; a second is taken while you’re on your stomach. The X-ray images are later examined for polyps and cancer.

While this procedure involves less of the body than a regular colonoscopy, one of the drawbacks is that if your HCP finds any irregularities, you’ll probably need a colonoscopy. Also, this test may not be covered by your insurance.

After the test: You may feel bloated or crampy.

Flexible sigmoidoscopy

Frequency: Once every five years

Preparation: A clear liquid diet the day before the exam may be required and you’ll need to drink a strong laxative. An enema (liquid injected into the rectum) may also be needed to be sure all waste is eliminated.

About the test: This procedure only takes 10 to 20 minutes and can be done without sedation. During a flexible sigmoidoscopy, your HCP inserts a thin, flexible, lighted tube with a video camera on the end through the rectum and into the lower part of the colon. Because the sigmoidoscope is only about two feet long, your HCP is only able to see a portion of the colon. If any irregularities are detected, your HCP may take a tissue sample for further examination. You’ll also need to have a colonoscopy to evaluate the rest of your colon.

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

Frequency: Once every year

Preparation for a FOBT: Your HCP may ask you to avoid taking nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen, naproxen, or aspirin a week before testing. You may also be asked to limit taking vitamin C and eating red meat for three days before the test.

About the FOBT: This screening test is different from a colonoscopy or sigmoidoscopy because it can be done in your own home. It relies on sending stool (poop) samples to your HCP’s office or a laboratory for testing. (A thin film of stool smeared onto special cards is usually required.) Detailed instructions on how to get the samples and return them are provided in a special kit. If laboratory tests show blood in the stool, you’ll need a colonoscopy to determine the cause of the bleeding.

Preparation for a FIT: Unlike the FOBT, there are no drug or dietary restrictions.

About the FIT: You’ll receive a test kit with detailed instructions for collecting and mailing stool samples to the lab. If the test shows hidden blood in your stool (in other words, blood you may not see when you have a bowel movement), you’ll need a colonoscopy.

Stool DNA test

Frequency: Once every one to three years

Preparation: No special preparation is required.

About the test: This screening test, which also requires stool samples, looks for irregular DNA that would appear in polyps or cancerous cells. You’ll again receive a kit for collecting and delivering stool samples. A colonoscopy will be necessary if the results show any irregularities.

Shield blood test

In July 2024, the U.S. Food and Drug Administration (FDA) approved a blood test for colorectal cancer screening. The Shield test, made by Guardant Health, is available by prescription for adults ages 45 and older who are at average risk for the disease. The test will reportedly cost $895 and is expected to be covered by Medicare. It works by finding the DNA that cancerous tumors release into the bloodstream.

Guardant Health recommends that the Shield test be taken every three years, like the FIT test. Experts have not yet made a formal recommendation on how often this blood test should be repeated.

The Shield test is a less invasive screening option than many, but it is not a replacement for colonoscopy, which remains the gold standard for colorectal cancer detection. That's because the Shield test is more likely to find cancer at a later stage, when it is more difficult to treat.

In clinical trials, the blood test correctly detected colorectal cancer in 83 percent of people confirmed to have the disease. These people had not yet developed symptoms. But it has more limited detection of precancerous growths and early stage colorectal cancers. Studies show it found only 55 to 65 percent of stage 1 cancers and missed up to 87 percent of advanced precancerous lesions, which can be found and removed during colonoscopy.

In trials, the Shield blood test also had a false positive rate of about 10 percent. (A false positive suggests that a disease is present when it really is not.) Anyone who tests positive—including false positives—would still need to have a follow-up colonoscopy to confirm their result.

Which test is right for you?

There’s no one approach to colorectal cancer screening that's right for everyone. Each test has its advantages and disadvantages. Your risk for colon cancer is an important factor in determining the best test for you. The stool-based laboratory tests and virtual colonoscopy are more likely to produce false-positive results than colonoscopy. That’s why it’s important to talk to your HCP about the risks and benefits of each option.

Regardless of which test you use, early detection increases your chances of successful treatment if cancer is found. If you’re at least 45 years old or at increased risk for colon cancer, speak with your HCP about setting up your screening plan today.