After age 50, regular screenings for colorectal cancer can be a lifesaver. But if you're too squeamish to undergo a colonoscopy, an annual fecal occult blood test, which looks for blood in your stool that can stem from precancerous polyps or colorectal cancer, can be a good alternative—if you do it right.

And that's not so simple: More than 80 percent of positive fecal occult blood test results could actually be false alarms, some research suggests. “There can be blood in your stool due to hemorrhoids or even a mild gastro­intestinal infection, and many drugs and medications can affect the test,” says David Greenwald, M.D., director of clinical gastroenterology and endoscopy at the Mount Sinai Hospital in New York City.

Here’s how to do the test right:

Watch what you eat. Two days before and the day of the fecal occult blood test, cut out all red meat, beets, broccoli, cantaloupe, carrots, cauliflower, cucumbers, grapefruit, horseradish, mushrooms, radishes, and turnips, which can all trigger false alarms.

Schedule dental work after the test. Anything that could cause your gums to bleed a significant amount should be avoided for at least three days before because you might swallow blood that could show up in your stool sample.

Limit OTC painkillers. Aspirin and related drugs, such as ibuprofen (Advil and generic) and naproxen (Aleve and generic), can cause microscopic amounts of gut bleeding that can show up in a sample. Try to avoid taking them for seven days before the test.

Avoid vitamin C pills. The nutrient interferes with the chemical reaction that occurs when blood is present, increasing the risk of a false negative result, so stay away from supplements, and food or drinks fortified with the vitamin, for about three days before a fecal occult blood test, Greenwald says.