- annual fecal occult blood tests (FOBT);
- flexible sigmoidoscopy (FS) every five years;
- the combination of FOBT and FS;
- double-contrast barium enema every five years; or
- colonoscopy every 10 years.
The two primary objectives of screening are to detect early, curable cancers and to detect and resect advanced adenomatous polyps before they turn cancerous. Relative to these two objectives, each of the screening options has advantages and limitations that clinicians and their patients need to consider when selecting a screening approach.
Annual FOBT Screening
FOBT is the only screening option that has been shown in randomized controlled trials to reduce both the mortality and incidence of colorectal cancer. The Minnesota FOBT Screening Trial reported in 1993 that screening asymptomatic individuals between the ages of 50 and 80 with annual rehydrated Hemoccult tests (Beckman-Coulter, Palo Alto, CA) and performing colonoscopy for those with a positive result, reduced the mortality from CRC by 33%. Investigators in this trial estimated that a screening program using their methods with 100% compliance would reduce CRC mortality by about 45% compared with that of a totally unscreened control group. The Minnesota Trial later showed that annual screening was substantially more effective in reducing mortality than was biennial screening. In addition, a program of annual screening reduced subsequent colorectal cancer incidence by 20%, presumably from detection and resection of advanced premalignant polyps. Although studies have shown that the sensitivity of a single FOBT test for detecting cancer is only 30%–50%, a program of repeated annual screening can detect up to 92% of all cancers, most of them at an early, curable stage. Other advantages of FOBT screening are its general availability and acceptability, and its very low up-front cost.The main disadvantages of FOBT screening are that frequent screening is required, it fails to detect many polyps (especially smaller ones) and some cancers (especially distal ones), and test specificity is relatively low—there are many false positive tests requiring patients without significant disease to undergo colonoscopy.
All of the large trials of FOBT screening used the guaiac-based Hemoccult test. The guidelines recommend that if Hemoccult tests are used for screening, two samples from each of three consecutive stools should be tested after following a diet free of red meat and peroxidase-rich fruits and vegetables. Rehydration, which increases the sensitivity of guaiac tests, is not recommended because it may interfere with the readability of the test and it increases falsepositivity. Newer guaiac-based and immunochemical FOBTs now are available that have greater sensitivity than standard Hemoccult tests, but also have acceptable specificity.The immunochemical tests, which now are being widely used in Japan and Australia, are still undergoing field testing in the US. These FOBTs are especially promising because they are specific for human globin and therefore are not affected by diet or medications. FOBT screening of a single stool sample obtained by digital-rectal examination—a common practice in primary care clinics—is now discouraged because such screening recently has been shown to be highly inaccurate. A positive screening FOBT should be followed by colonoscopy, no exceptions.