Source: A stool DNA test (Cologuard) for colorectal cancer screening. JAMA 2014;312:2566-2567.

Recent guidelines issued by the American Cancer Society and American Society of Gastroenterology recommend colonoscopy as the preferred screening method for colon cancer, but wisely include the philosophy, “The best colon cancer screening test is the test you can get done!” — reflecting the relative reticence shown by many Americans to undergo colonoscopy. Computed tomography colonography compares very favorably to colonoscopy, yet many insurers are not willing to pay for it.

Stool DNA testing (trade name Cologuard) is based on the observation that colon cancers consistently shed cells that contain cancer markers in the stool (example names of such markers include KRAS, NDRG4, BMP3). In the most recent iteration of stool DNA testing kits, an assay for human hemoglobin is also included (eliminating the need for dietary restriction prior to stool testing).

The process if fairly simple: a collection kit is sent to the patient’s home, the specimen is mailed back, and if either the colon cancer markers or the human hemoglobin test return positive, the patient needs follow-up colonic evaluation (e.g., colonoscopy). Some patients (and clinicians) may feel so reassured by negative stool DNA testing that they feel comfortable to stop screening at that point. Unfortunately, even though stool DNA testing is very sensitive for colon cancer compared to colonoscopy (sensitivity = 92%), it is much less sensitive for precancerous lesions. Additionally, some false-positives occur with stool DNA testing.

Despite these limitations, stool DNA testing provides a viable way to do colon cancer screening for persons unwilling to undergo other screening methods. Negative stool DNA screening will be so reassuring to some patients that they will elect not to pursue further, more definitive screening.