Studies using the three-card, take-home method of FOBT screening repeated over a number of years have demonstrated mortality reductions of up to 33%. Conversely, there are no large studies documenting a benefit from the performance of a single FOBT at the time of an office rectal exam, and this screening method is not recommended by any national body. That point notwithstanding, nearly one quarter of clinicians reported in a recent survey that they use the office FOBT as the sole method of screening for some or all of their patients.
All positive FOBTs should be followed up with colonoscopy. Data from Medicare indicate that fewer than half of all positive FOBTs in this population receive this recommended follow-up.
Flexible sigmoidoscopy (FSIG): FSIG involves the insertion of a flexible endoscope into the rectum of a patient. Through the use of fiberoptics, the interior walls of the rectum and colon are viewed. Small polyps or other lesions can be removed during the procedure, and larger lesions biopsied. FSIG is relatively easy to perform and has a low rate of serious complications.
The test is commonly done by a wide variety of providers, including internists, family clinicians, general practitioners and some specially trained nurse practitioners and physician assistants. Preparation usually consists of one or two enemas on the day of the procedure.
A well-performed FSIG will reach the splenic flexure, thus allowing visualization of approximately the lower third of the colon. Because a higher proportion of cancers occur in the descending colon (approximately 50%) and because many cancers in the transverse or ascending colon are accompanied by concurrent cancers or adenomas in the left colon, FSIG is estimated to lead to the detection of 70%-75% of cancers present at the time of screening. In addition, the predictive value of a normal FSIG is also high; therefore, this test has a recommended screening interval of every five years.
Because of the indirect nature of FOBT and the limited reach of the sigmoidoscope, some authorities recommend that annual FOBT be combined with FSIG every five years. There is evidence to support a benefit from the combination of these tests over either of the tests alone. The ACS guidelines for the early detection of CRC specifically state: “Flexible sigmoidoscopy together with FOBT is preferred compared with FOBT or flexible sigmoidoscopy alone.”
Detection of an adenoma or any suspicious lesion at the time of FSIG should lead to immediate referral to colonoscopy.
Colonoscopy: Colonoscopy is performed with a flexible endoscope similar to a sigmoidoscope. The colonoscope is significantly longer than a sigmoidoscope and allows the endoscopist to view the lumen of the entire colon. Mastery of colonoscopy requires significantly more training and experience than does sigmoidoscopy, and the vast majority of colonoscopies are performed by gastroenterologists and surgeons.
As with FSIG, lesions can be biopsied or removed during colonoscopy. Given the relatively prolonged period from benign tissue to adenoma development to transformation to cancer, the recommended screening interval following a normal colonoscopy is 10 years.
Colonoscopy also requires a more extensive bowel prep, usually including low-bulk or liquid diets and cathartics. Most colonoscopies in the United States are performed using conscious sedation, adding an additional layer of complexity.
Although colonoscopy is viewed as the gold standard for CRC screening, the test is not perfect. Several trials have shown that even experienced colonoscopists fail to detect up to 10% of significant lesions. In addition, major complications (bleeding, perforation, and rarely death) occur in approximately one in 1000 screening colonoscopies.
Nevertheless, visualization of the entire colon, combined with the diagnostic and therapeutic potential of the test, make colonoscopy the recommended follow-up procedure for a positive result with any other screening modality.
Double-contrast barium enema (DCBE): DCBE involves the instillation of barium into the rectum via an anal tube. This is followed by colon insufflation with air. X-rays are taken in a number of positions, and these are examined for the presence of abnormalities in the colon lumen. Abnormalities are assessed via colonoscopy.
DCBE has been shown to be fairly sensitive for the detection of large polyps and cancers. For a variety of reasons, however, the use of this technique as a screening modality for CRC cancer has waned in recent years. While it is a less than perfect tool, DCBE may be the only screening that some individuals are willing to undergo. It can be useful when cooperation with (or access to) colonoscopy is limited.