Stool Test for Colon Cancer Screening

Stool testing for occult blood has long been used as a method for colon cancer screening. Recently a stool test  (Cologuard -Exact Sciences Corporation) combining testing for human blood and for DNA mutations of cancer cells in the stool has been approved by CMS for use for colon cancer screening in average risk patients. It is not recommended for patients with a history of colon polyps, colon cancer, a family history of colon cancer, or inflammatory bowel disease.The advantage of stool testing is that it is noninvasive and does not require a bowel preparation. Also no dietary restriction is required as the newer tests are able to detect human hemoglobin. However there are several disadvantages to the stool tests compared to colonoscopy which is still regarded as the gold standard for colon cancer screening.

A trial of stool DNA for colon cancer screening involving 9989 participants was published in 2014 (NEJM 2014; 370:1287-1297). The stool DNA test was able to detect colon cancer in 92.3% of participants with cancer. The false positive rate for the stool DNA test was 13.2%. The stool DNA test was not able to detect 58% of advanced precancerous polyps and 83% of non-advanced polyps. The rationale for colon cancer screening is to detect precancerous polyps and remove them before they progress to colon cancer. Thus the stool DNA test is not as effective as colonoscopy in detecting precancerous colon polyps and colonoscopy is required to remove polyps. Therefore the stool DNA test is approved by CMS for screening every 3 years for a negative test compared to every 10 years for a negative colonoscopy. Also a person with a positive stool DNA test result would need to have colonoscopy for further evaluation. Many third-party payers will not cover colonoscopy as a screening exam if a person has already had a stool DNA test. For these reasons NGS physicians recommend colonoscopy as the preferred test for colon cancer screening.