Colonoscopy is a procedure in which a scope is inserted into the rectum, and the doctor performs a visual exam of the entire colon. If he or she encounters an abnormal growth, it can often be removed or biopsied while undergoing the procedure. It is one of the most common ways to diagnose colon pathology.

A good screening test is one that can identify an illness before any symptoms develop, or at least before they become obvious. Furthermore, a screening test should result in a decline of serious complications, including death, over time.

Although colonoscopy is widely used as a screening test, its effect on the risks of colorectal cancer and death from the disease is unclear. During my medical training, colonoscopies, sigmoidoscopies, and fecal testing for blood were recommended as screenings for colon cancer.

I was taught that colon cancer starts with small growths called polyps that have potential to become malignant. If the polyps are identified at a very early stage, they can be safely removed with little or no complications. Removal of a small polyp decreases the incidence of developing colon cancer years later.

Sigmoidoscopy is a similar procedure in which a scope is inserted into the rectum and the sigmoid colon — the lower part — is visualized for abnormalities. A colonoscopy is a more thorough exam than a sigmoidoscopy. Sigmoidoscopies are not performed as frequently today as they were in the past, even though the medical literature shows benefits from the procedure for preventing colon cancer death.

However, to assess the entire colon, colonoscopy is superior to sigmoidoscopy.

More than 15 million colonoscopies are performed each year in the U.S. Patients are routinely told by their primary care doctors that a screening will identify early malignant growths that when removed will lower the person’s risk for developing cancer. I wish it were that easy.