The accuracy of this test depends a lot on you

It’s widely known that colonoscopy is the most effective way to help prevent cancer of the colon and rectum. What you may not know: The results of some colonoscopies are more accurate than others.

Important recent finding: When researchers reviewed the medical records of nearly 5,000 people diagnosed with colorectal cancer, 8% of the cases — or about one in 13 — had malignancies that were not detected during colonoscopies performed within the prior three-year period.

Of course, the skill of the doctor you choose to perform your colonoscopy affects whether polyps (growths that are typically benign and can be removed before they turn malignant) and/or cancers are found. But to a surprising extent, the steps you take before your colonoscopy also greatly affect the test’s accuracy.


For most people over age 50, colonoscopy should be performed every three to five years if polyps have been found and up to every 10 years if no polyps have been found.* Perhaps the most dreaded part of the test, however, is the bowel prep.

If the bowel is not cleansed properly, it increases the likelihood that growths will be missed and that the procedure will take longer than it should, thus increasing the possibility of complications, such as perforation of the colon or rectum wall and post-procedure abdominal discomfort. If the test results are questionable, you may also need to repeat the test sooner.

To give yourself the best odds of an accurate colonoscopy…

Secret 1: Start preparing early. Review your doctor’s instructions on bowel prep one week before the procedure, if possible. You may be told to temporarily avoid certain medications, such as iron-containing drugs or supplements that can color the intestines and make preparation more difficult. If you are taking medications that increase bleeding risk — for example, blood thinners such as clopidogrel (Plavix) or warfarin (Coumadin) — you should contact your doctor or the colonoscopist for instructions.

If you take medication for a chronic illness such as heart disease, ask your doctor whether you should take it up to and including the day of the procedure. People with diabetes may need to adjust their medication to keep their blood sugar levels normal while they are following dietary restrictions.

Beginning two to three days before the colonoscopy, many doctors advise you to avoid undigestible fiber from foods such as nuts, seeds, corn and bran cereal. Little pockets of residue from such foods may linger in the intestine, obscuring the doctor’s view.

Secret 2: Be clear on what foods you can eat. For years, the standard instruction was to take nothing but clear liquids by mouth the day before a colonoscopy. But there is increasing evidence that a diet that includes fiber-free solid foods is not only easier to follow, but also may result in better cleansing.

A 2010 study in the journal Endoscopy found that patients tolerated the laxative better and the bowel was cleaner when they followed a fiber-free diet that included scrambled eggs, cheese and white bread, compared with a clear-liquid diet. Theory: A liquid diet may decrease intestinal contractions and slow down bowel activity, while a bit of solid food keeps the bowel active so it can empty more completely.

Don’t take this step on your own, though — ask your doctor if it’s OK to eat light, fiber-free meals the day before your colonoscopy.

Secret 3: Discuss your options for a laxative. The most important part of bowel prep is taking a strong laxative, usually starting the afternoon or evening before you’re scheduled for the procedure.

This is typically an unpleasant-tasting prescription preparation, such as polyethylene glycol (PEG), dissolved in an electrolyte solution containing sodium, potassium and other minerals. You may also be instructed to take laxative pills, usually bisacodyl (Dulcolax), at some time during the prep.

Not long ago, the standard approach called for a “high-volume” solution — four liters (more than a gallon) of liquid consumed over a period of several hours. Now, many doctors prescribe a “low-volume” solution — two liters versus four — that most people find easier to consume.

Caution: The low-volume approach usually works well but may fail to cleanse the bowel adequately if you are prone to constipation or take medication that slows down digestion, such as an opioid for pain or a tricyclic antidepressant, such as amitriptyline (Elavil).

If you are still passing brown stool when it’s time to leave home for your colonoscopy, let the nurse know when you arrive at the colonoscopy center. The doctor may ask you to use an enema for further cleaning before starting the colonoscopy.

Secret 4: Ask your doctor about splitting the laxative dose. Whether you go the high-volume or low-volume route, when you take the purgative can make a crucial difference.

Most doctors used to advise drinking all the solution the night before the procedure. But many now counsel dividing the dose — half the night before and half four to five hours prior to the colonoscopy, even if you have to drink it at 3 am. A 2009 study rated the bowel significantly cleaner when less time elapsed between preparation and procedure.

Here’s why: The laxative cleans fecal matter out of the colon, but over time, thick digestive mucus will descend from the small intestine, accumulate and obscure the doctor’s view of the part of the large intestine where many cancers develop. Studies have shown that seven to eight hours after taking the laxative, the ascending bowel starts to become obscured by mucus. By 15 to 16 hours, it’s often covered with mucus.

Secret 5: Make sure you’re not alone during a bowel prep. It’s rare, but some people get shaky or pass out during bowel prep. This may result from the abdominal cramping, vomiting, diarrhea and/or dehydration that can occur in some patients, so have someone with you overnight, particularly if you’re an older adult and/or frail. Drink plenty of fluids to help prevent dehydration. Your doctor will tell you when to stop drinking and eating prior to the colonoscopy.

Secret 6: Schedule early in the day, if possible. Several studies have shown that some physicians detect fewer polyps as the day wears on, possibly due to fatigue. For this reason, you may want to schedule the procedure for earlier in the day, although time of day makes far less difference than the doctor’s skill.


Gastroenterologists as well as some family physicians and internists perform colonoscopies, but gastroenterologists typically have more training and experience doing the procedure.**

However, the number of polyps a doctor detects in his patients is a much better measure of competence than his specialty. Look for a doctor who finds growths in 20% or more of patients.

If a doctor does not find precancerous polyps in this percentage of his screening patients, these patients are up to 10 times more likely to develop cancer before their next colonoscopy. If your doctor can’t or won’t tell you the number of growths he finds in his patients, consider looking elsewhere.

Research also shows that doctors who spend more time find more growths. For example, doctors should take at least six minutes to withdraw the scope (that’s when the actual inspection takes place). Although taking more time doesn’t guarantee a quality examination, it’s fair for patients to tell a doctor that they’ve heard that taking longer increases polyp detection and that they’re expecting a slow, careful exam and documentation of how long the procedure took.

*Some people, including those with a family history of colorectal cancer, may be advised to begin regular colonoscopies sooner or get them more often — speak to your doctor.

**To find a gastroenterologist near you, consult the American College of Gastroenterology at (click on “Patient Information” under the “Patients” tab).