Colonoscopy is still the best way to detect colon cancer

Q: I heard about a new home test that detects colon cancer. Is it a good alternative to colonoscopy?

A: The new test appears to be an advance, but I don't think it's as good as colonoscopy. Particularly for people who are at higher risk for colon cancer, I regard colonoscopy as the best test.

Colon (or colorectal) cancer lies in the wall of the colon. It can cause painless bleeding. The amount of blood can be so small ("occult blood") that it isn't visible in the bowel movement, but it can be detected by chemical tests. Performing these tests for blood in the bowel movement - fecal occult blood tests - has been a traditional way of trying to catch colon cancer early.

However, colonoscopy remains the best way for catching colorectal cancer early. This test, done under sedation, uses an instrument to look inside the colon.

A similar procedure, flexible sigmoidoscopy, looks at about half of the colon and doesn't require sedation. These tests not only can see cancer, but also abnormalities (adenomas) that can become cancerous in the future. The tests also allow the adenomas and some very early cancers to be removed during the test.

In August 2014, the FDA approved Cologuard, a new test for colorectal cancer. The Cologuard test is done at home. You collect a stool sample and mail it to a laboratory for analysis. Like the fecal occult test, it checks for blood in stool. But the new test also looks for abnormal DNA.

Colon cancers develop because genes in the cells lining the colon have developed mutations. The abnormal DNA caused by these mutations, like occult blood, is shed in the bowel movement. That abnormal DNA can now be detected by special tests.

One large study of nearly 10,000 people found the new test to be very effective. People in the study were given a fecal occult blood test, the Cologuard test and a colonoscopy.

Cologuard detected 92 percent of the cancers that colonoscopy found; the fecal occult test detected 74 percent. Cologuard also found 69 percent of the polyps that were considered most likely to turn into cancer. The fecal occult test found 46 percent.

On the down side, the Cologuard test produced more false positives than the fecal occult test. A false positive test says there is a polyp or cancer when none actually exists - and that requires further investigation with a colonoscopy to confirm or rule out the diagnosis.

The new Cologuard was approved for screening in people ages 50 to 85 who have an average risk of colon cancer. It is not meant as a substitute for colonoscopy if you have a greater-than-average risk. This includes people with:

• A history of polyps or prior colorectal cancer;

• A strong family history of colorectal cancer;

• Ulcerative colitis or Crohn's disease.

Cologuard offers another option for colorectal cancer screening. But right now, colonoscopy still remains the best screening method. It is recommended once every 10 years for people age 50 and over who have an average risk of colorectal cancer.

• Dr. Komaroff is a physician and professor at Harvard Medical School. For questions, go to AskDoctorK.com.