Details of Preparing for a Colon Screening Test

doctor comforting patient in exam room

Regular colon testing is important because it can prevent cancer or find it early when it is easier to treat. This is because some polyps (growths) can be found and removed before they have the chance to turn into cancer. The American Cancer Society recommends colon cancer testing (there are several acceptable methods) for everyone starting at age 50, though some people with certain risk factors may need to start at a younger age.

But the idea of testing and the preparation required for many of the tests (often called bowel prep) is unpleasant for many people and may discourage them from getting recommended tests. The prep may include eating a special diet, drinking up to a gallon of a liquid laxative, and sometimes enemas to clean out the colon.

Knowing what to expect and correcting any misperceptions can take away some of the fear and restore your sense of control over the experience.

Which tests need a bowel prep?

Cancer screening is the process of looking for cancer in people who have no symptoms of the disease. The tests that can be used to screen for colon cancer can be divided into 2 groups – those that can find both polyps and cancer, and those that mainly find cancer.

The tests that mainly find cancer include the fecal occult blood test (FOBT) and the fecal immunochemical test (FIT). They examine the stool to look for signs of cancer. They do not require a bowel prep. But they are not as good as the other group of tests at finding polyps. And if the results from the FOBT or FIT are positive, a more invasive test such as a colonoscopy will probably be needed.

Several tests can find both polyps and cancer. They use either a scope inserted into the rectum or x-rays to look inside the colon. The colon has to be as empty as you can get it so the doctor can clearly see inside. That’s why the bowel prep is so important.

The tests that require the bowel prep include colonoscopy, CT colonography (also called virtual colonoscopy), and double-contrast barium enema. Flexible sigmoidoscopy requires a prep, too, but it isn’t always as extensive as the one used for other colon tests. Your doctor can help you figure out which test is most appropriate for you. You can read more about these tests and what they involve.

What to expect from the prep

Undergoing a bowel prep means you have to take medicine that makes you go to the bathroom a lot. Your doctor will give you instructions. Make sure you read them carefully, at least a week ahead of time. This will give you time to shop for supplies, including laxatives or enemas you may need to get from a pharmacy. It will also give you time to call the doctor’s office to ask questions if any of the instructions are unclear. You may also be instructed to stop taking certain over-the-counter or prescription medications as long as a week before the test.

You will probably be told to start the prep a day or 2 before the test. You may need to follow a special diet. For example, your doctor may tell you to stay on a clear liquid diet for at least a day before the exam. That may mean you can have only clear liquids such as water, apple or cranberry juice, clear broth, ginger ale, sports drinks, plain coffee or tea (sugar and sweeteners are OK, but not milk or cream), and gelatin. You may be told to avoid red or purple sports drinks or gelatin because the food coloring in them can be mistaken for blood in the colon. You’ll probably be told not to eat or drink anything at all after midnight the night before your test.

The day before the test, you’ll begin taking very strong laxatives. As soon as you start the laxatives, you’ll need to stay near a bathroom. This will likely mean canceling any plans for the day, including work. You may spend much of the night before the test in the bathroom. And you may be instructed to use an enema the morning of the test.

Many people consider the bowel prep the worst part of getting a colon screening test. Paula Alan had her first colonoscopy in 1988 at age 50. She had to drink a gallon of liquid laxative she calls “revolting.” She’s had several more colonoscopies over the years, but says the prep has gotten easier because of newer methods that require drinking less liquid. During each of Alan’s colonoscopies, the doctor found and removed polyps. Alan says even though she doesn’t like the prep, she follows her doctor’s recommendation for screening because she doesn’t want to get colon cancer.

Having the test

The various tests that screen for colon cancer and polyps may be done in a doctor’s office, a hospital, a clinic, or a surgery center. The tests are typically done in a private room, and the doctors and nurses will treat you professionally, and take care to protect your privacy.

Depending on the test, you may be given medicine to make you relaxed and sleepy. If so, you will not be allowed to drive afterward, so someone else must come with you and drive you home. Most doctors require that it be someone you know, and won’t allow you to take the bus or a taxi. You’ll also have to plan to stay home the day of the test until the drugs wear off.

Most people don’t feel pain during any of the tests, but may feel cramping or discomfort afterward because of air that was puffed into the colon during the test, to keep it open for the doctor to examine. Alan says during each of her colonoscopies, once she was asleep, she was blissfully unaware of anything. She never felt any pain or discomfort during the tests or after waking up.

If they find something

If a polyp is found during a colonoscopy or sigmoidoscopy, your doctor will probably remove it during the test. If your doctor sees a polyp too large to be removed or a tumor or other abnormality, a biopsy will be done to check for cancer or pre-cancer cells.

If you had a sigmoidoscopy and polyps were found, you will need to have a colonoscopy later to check for polyps or cancer in the rest of your colon. Because the colonoscopy has to be done anyway, some doctors wait until then to remove any polyps found.

If you had a CT colonography or double-contrast barium enema and polyps or suspicious areas were found, you will need to have a colonoscopy later to check them out.

Screening saves lives

Removing polyps can help prevent colon cancer from ever starting. And cancers found in an early stage, while they are small and before they have spread, are more easily treated. Nine out of 10 people whose colon cancer is discovered early will be alive 5 years later. And many will live a normal life span.

But all too often people don’t get any of these screening tests. Then the cancer can grow and spread without being noticed. Early on, colorectal cancer doesn’t usually cause any changes that are noticed (symptoms). In most cases, by the time people do have symptoms the cancer is advanced and very hard to treat.

Regular screening is the most reliable way to find these cancers in the early stages. Ask a doctor about the best screening plan for you.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master's-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

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