Screening for colorectal cancer is a proven lifesaver, yet about a third of Americans over age 50 are not getting tested at all. One reason may be an overemphasis on colonoscopy as the screening test of choice in this country, which many people shy away from due to fears about the preparation process or the procedure itself. We spoke with Steven Jacobsohn, M.D., Professor Emeritus of Gastroenterology at the University of California, San Francisco Medical Center and a member of our editorial board, about the annual fecal immunochemical test (FIT), which screens for human blood in the stool.

FIT is one of the top-tier screening options recommended by the U.S. Multi-Society Task Force on Colorectal Cancer (MSTF) for people at average risk of colorectal cancer. (The MSTF represents the American College of Gastroenterology, the American Gastroenterological Association, and the American Society for Gastrointestinal Endoscopy.) Other colorectal cancer screening methods recognized by the MSTF include sigmoidoscopy and virtual colonoscopy, though they are considered lower-tier options and are not discussed in this interview.

Wellness Letter: Why is FIT a good colorectal cancer screening test?

Dr. Steve Jacobsohn: Colorectal cancer is the second leading cause of death from cancer in the U.S. but is highly preventable. Regrettably, only about 65 percent of the recommended population gets screened. Screening with FIT has been shown to decrease mortality from colorectal cancer and has increased screening compliance. The test is inexpensive, covered by most insurance, and easy to do: It involves taking a stool sample at home and sending it to the lab. It avoids having to do an invasive test like colonoscopy unless the FIT result is positive, after which a colonoscopy would be needed. Since FIT only tests for human blood, no dietary restrictions are necessary beforehand. For example, eating foods that contain other animal blood, such as red meat, won’t affect the results. Also, because the test is specific for colon and rectal lesions, the results will not be affected by blood coming from a source above the colon, such as the stomach or small intestine.

WL: Is FIT as accurate as colonoscopy?

SJ: Both FIT and colonoscopy are considered effective in detecting colorectal cancer. Studies are underway to compare their value in reducing mortality. Prior to research showing the value of FIT screening, colonoscopy was considered the screening test of choice by several influential gastrointestinal societies.

The MSTF places both tests in the top tier of screening options for people at average risk of colorectal cancer. In many countries, including Canada, a positive FIT result is required before a colonoscopy can be performed in average-risk individuals. There is no current “gold standard,” a concept that has not yet been acknowledged by all gastroenterologists and primary care doctors. Fortunately, doctors’ attitudes are changing—and with more of them accepting and understanding the value of FIT in screening, the percentage of people who are undergoing screening with this test has increased.

FIT has moderate to high sensitivity and specificity (meaning it has few false-positive and false-negative results) in detecting colorectal cancer. It is more accurate in detecting advanced polyps than smaller polyps, which are more likely to be detected by colonoscopy. But it’s important to note that only about 6 percent of all polyps go on to become cancerous. FIT screening should be done annually using kits prescribed by your doctor. The most accurate results are those evaluated in labs by automation or by well-trained certified personnel. Ask your doctor which specific FIT is being used. It should be one that has been carefully studied, such as the OC-Sensor, which is one of the most sensitive tests available. Over-the-counter FITs have not been adequately evaluated for quality and should not be used.

WL: What are the downsides—and possible upsides, if any—to using FIT, as opposed to colonoscopy?

SJ: On the downside, because FIT detects blood coming from the colon and rectum, the result will be positive if bleeding is caused by a problem other than a significant colon or rectal lesion—for example, from a hemorrhoid. Therefore, the test should not be done at a time when rectal bleeding is visible (on toilet paper after a bowel movement or in stool in the toilet) or during a menstrual cycle (menstrual blood may contaminate the test results). Note that anyone with rectal bleeding should have some type of endoscopic evaluation.

Another downside: Doing FIT does not always preempt the need for a colonoscopy. Polyps found during colonoscopy can be removed at that time, so with that approach, you would just have the one screening test. If the FIT result is positive, however, then colonoscopy will be needed within six months. Then again, since the majority of polyps never become malignant, it still remains to be seen if removing polyps during a colonoscopy is more effective at reducing the risk of cancer or mortality compared with FIT screening, which predominantly detects cancer and polyps that are more likely to become cancer, because these are more likely to bleed.

As noted above, FIT screening has to be done every year (though every two years is also acceptable), while colonoscopy need not be repeated for 10 years if adequate colon cleansing has been accomplished and no significant lesions are found. On the other hand, a potential advantage of FIT compared with colonoscopy is that annual FIT screening might detect a significant lesion that could have developed after a negative colonoscopy done within the last 10 years, or it may detect a lesion that might have been missed on a colonoscopy.

WL: How does FIT compare with other at-home stool tests, such as the fecal occult blood test (FOBT) and FIT-DNA (Cologuard)?

SJ: FOBT is a stool test for blood that was in use many years before the advent of FIT. Ideally, however, it should no longer be used because it produces more false positives and false negatives than FIT. Because FOBT is not specific for blood coming from the colon or rectum, a restricted diet is necessary before testing; unlike with FIT, certain foods, supplements, and medications can affect FOBT results.

Cologuard is a much-advertised screening test—you may have seen the commercials—that the MSTF ranks below FIT as a tier 2 option. Approved for use every three years, it combines FIT with a test for DNA markers associated with some colon cancers. A positive result, though, does not indicate if this was from FIT or DNA markers. If it was due just to FIT (and not to DNA markers), this is a very expensive way of getting the result compared to doing FIT alone since Cologuard is significantly more expensive than FIT (about $650 versus $20). Also, if it is the FIT part in any given patient that yields a positive result (and not the DNA markers), then a negative test would result in inadequate screening intervals, so cancers could be missed by not doing the more-frequently recommended FIT screenings. At this time, proof is lacking that Cologuard is better than FIT when FIT is done on an annual basis. And in one study, Cologuard produced twice as many false positives (meaning that the tests were positive though no cancers were present) as FIT.

WL: What is your advice for screening people ages 75 and older?

SJ: Colorectal cancer screening is recommended up to age 75. For people ages 76 to 85, the MSTF suggests that starting or continuing screening should be individualized and based on prior screening history, life expectancy, comorbidity, colorectal cancer risk, and personal preference. For those over age 85, screening is not recommended. This advice was updated in November 2021.

NOTE: The American Cancer Society provides information about the safety of colorectal screening during the Covid pandemic here.