RODALE NEWS, EMMAUS, PA—Although colonoscopy exams prevent many colon cancer deaths, the procedure does not usually catch cancers on the right side of the colon in time to prevent death, according to a study published in the journal Annals of Internal Medicine. Colonoscopies were previously credited with being 90% effective in preventing colorectal cancer deaths. The imaging test now appears to be about 60 to 70% effective preventing death in left-side colon cancers, where most develop.

THE DETAILS: Lead author Nancy Baxter, MD, PhD, colorectal surgeon at St. Michael's Hospital in Toronto, and researchers compared data from about 10,300 patients who died from colorectal cancer with about 51,500 patients who did not. They found the colonoscopy greatly reduced the likelihood of death when the cancer developed on the left side of the colon, but didn’t have an impact when the cancer grew on the right side. That could be because polyps, growths in the colon that can become cancerous, grow differently on the left side versus the right, says Dr. Baxter. “Polyps on the left side look like mushrooms,” she explains. “They stick out and have a nice stalk; they’re easier to take off. On the right side, they are flatter, harder to see and remove.” Another factor that may be lessening the effect of colonoscopy: Baxter says doctors performing the procedure have to pass the imaging scope through the left colon to get into the right side, and they may not always send the scope all the way through. Or, they may pull the scope out sooner than the recommended 6 to 8 minutes, missing some polyps. The standard preparation regimen—drinking clear liquids, taking laxatives, and fasting to flush the colon clean for better visibility during the scope—could be another setback in detecting right-sided cancers. It tends to leave a sticky substance coating the surface of the right colon.

WHAT IT MEANS: While this is certainly a wake-up call for doctors screening for the disease, you shouldn’t be tempted to skip recommended colonoscopies. That’s because, aside from cervical cancer screening, no other screening is as effective at preventing cancer deaths than colonoscopy. “Colonoscopy remains a highly effective screening tool for prevention of deaths from left-side colon cancers and is the best diagnostic test for the colon available,” Baxter says.

To cut your colon cancer risk, exercise regularly, keep at a healthy weight, cut out as much red meat from your diet as you can, and take vitamin D and calcium supplements. When it’s time for a screening, though, one of the most important things you can do is really ace your prep. Although the process of downing laxatives, sipping clear liquids, and then fasting is not generally pleasant, having a cleared-out colon is key for spotting polyps that can lead to cancer.

Here are some prep tips to make your bowels clean as a whistle the day the doctor goes in for a look-see: