The chance of a false positive test can be reduced by not consuming red meat, vitamin C and iron—avoiding cantaloupe, broccoli, parsnips, cauliflower, mushrooms, potatoes, cucumbers, red radishes and horseradish—for 24 hours before the test. A test called HemoQuant has fewer false positive results.
    The sigmoid colon and rectum should be examined with a flexible or rigid tube (sigmoidoscopy) every three to five years after age 50 (after two negative sigmoidoscopies one year apart). Colonoscopy will examine the entire colon. It takes about five years for an adenomatous polyp to grow to the size where a cancerous change may occur.
    Measuring the level of the carcinoembryonic antigen (CEA) in the blood can detect a few early cancer cases, but it is not a good test for screening .
    Soon genetic studies of the genes mentioned previously will improve screening for people at high risk for developing rectal cancer. Recent research has identified a gene responsible for an inherited form of colon cancer. It is hoped that a blood test will soon be available to identify individuals at high risk.
    Screening is costly but may be effective. Finding colorectal cancer before it invades tissues or spreads to lymph nodes will improve the cure rate. The Canadian death rate from this cancer fell 5.5 percent from 1973 to 1985, but there are still 6,300 deaths per year.