Colonoscopy

Colonoscopy may be used as a screening method without stool testing or FS.  The current ACS recommendation for asymptomatic adults over 50 years of age is that colonoscopy, if done as the primary screening method, should be repeated every 10 years.  However, the precise interval needed to maximize prevention of colorectal cancer is not known.

There are no randomized controlled trials to date evaluating the impact of screening colonoscopy on CRC incidence or mortality in average risk individuals.  However, there is evidence to support its effectiveness.  Colonoscopy was employed in the clinical trials of FOBT, which revealed that screening decreased mortality.  It is thought to be at least as good as FS, as it examines the entire colon and not just the distal segment. Colonoscopy has been shown to reduce the incidence of CRC in persons with adenomatous polyps in two cohort studies. (5)

Performing the Test

Colonoscopy must be performed by a trained practitioner.  Colonoscopy involves passing a colonoscope into the rectum.  The colonoscope is used to examine the rectum and the entire colon, to determine whether colorectal polyps and/or cancer are present.

The procedure requires a complete bowel preparation with a strong laxative the day before the procedure, usually preceded by a diet of clear liquids for a day or two. Since colonoscopy usually includes the use of a sedative, the patient needs someone to take him/her home at the end of the test.

The test takes about 30 minutes.  It may take longer if visualization is difficult or if multiple polyps must be removed or biopsies must be done.  There may be some discomfort, pressure or cramping.  The physician is usually able to remove any polyps he or she finds and to take biopsies of any suspicious lesions during the colonoscopy.

Risks of the Test

Colonoscopy does involve greater risk, cost and inconvenience than other CRC screening tests.  Risks include perforation and major bleeding, as well as complications from the use of a sedative.  The USPSTF notes that risk of major complications, including significant bleeding, is about 2 to 3 per 1,000 for screening colonoscopies, with higher risk for therapeutic procedures.  Different studies of diagnostic colonoscopies have shown rates of bowel perforation ranging from 3 to 61 in 10,000 examinations.  The risk-benefit ratio should be discussed in the framework of shared decision-making, outlined in the previous section. (5)

Interpreting the Test
Hyperplastic, non-neoplastic polyps account for 10-30% of polyps detected on examination and in most cases are not clinically significant.  Other benign growths include lipomas and mucosal tags.  Adenomatous polyps comprise 50-70% of all polyps found at the time of colonoscopy and are usually neoplastic or pre-malignant. One-third of these lesions are found proximal to the splenic flexure. (12)

If one or more adenomas are detected, a repeat colonoscopy sooner than ten years is usually recommended.  The date of this follow-up surveillance depends on the number, size, and histology of the adenomas found.  Common intervals are three and five years. (6)

If the results of a screening colonoscopy are normal, the test should be repeated every 10 years in average risk patients.  The ten-year interval is based on the sensitivity of colonoscopy and the rate at which advanced adenomas develop. (5)  Screening intervals for higher-risk patients are beyond the scope of this workbook.