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Colorectal cancer screening: Choosing the right test

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DISCUSSING THE OPTIONS

Options in colorectal cancer prevention and detection
Currently approved screening options and intervals between examinations are summarized in Table 1.

Stool studies such as FOBT and FIT do not reliably detect cancer precursors such as adenomas and serrated neoplasms. If an FOBT is positive, follow-up diagnostic colonoscopy is required. Unlike screening colonoscopy, diagnostic colonoscopy requires a copayment for Medicare patients, and this should be explained to the patient.

FIT and FOBT detect hemolyzed blood within a stool sample, FOBT by a chemical reaction, and FIT by detecting a globin-specific antibody. Colorectal cancer and some large adenomatous polyps may intermittently bleed and result in occult blood in the stool, iron deficiency anemia, or hematochezia.15

Fecal occult blood testing

Historically, FOBT was the stool test of choice for screening. It uses an indirect enzymatic reaction to detect hemolyzed blood in the stool. When a specimen containing hemoglobin is added to guaiac paper and a drop of hydrogen peroxide is added to “develop” it, the peroxidase activity of hemoglobin turns the guaiac blue.

Screening with FOBT involves annual testing of 3 consecutively passed stools from different days; FOBT should not be performed at the time of digital rectal examination or if the patient is having overt rectal, urinary, or menstrual bleeding.

Dietary and medication restrictions before and during the testing period are critical, as red meat contains hemoglobin, and certain vegetables (eg, radishes, turnips, cauliflower, cucumbers) contain peroxidase, all of which can cause a false-positive result. Waiting 3 days after the stool sample is collected to develop it can mitigate the peroxidase activity of vegetables.16 Vitamin C inhibits heme peroxidase activity and leads to false-negative results. Aspirin and high-dose nonsteroidal anti-inflammatory drugs can promote bleeding throughout the intestinal tract.17

In randomized controlled trials,18–21 screening with FOBT reduced colorectal cancer mortality rates by 15% to 33%. The 30-year follow-up of a large US trial22 found a 32% relative reduction in mortality rates in patients randomized to annual screening, and a 22% relative reduction in those randomized to screening every 2 years. Despite the many possibilities for false-positive results, the specificity for detecting cancer has ranged from 86.7% to 97.3%, and the sensitivity from 37.1% to 79.4%, highlighting the benefit of colorectal cancer screening programs in unscreened populations.23–26

FIT vs FOBT in current practice

FIT should replace FOBT as the preferred stool screening method. Instead of an enzymatic reaction that can be altered by food or medication, FIT utilizes an antibody specific to human globin to directly detect hemolyzed blood, thus eliminating the need to modify the diet or medications.27 Additionally, only 1 stool specimen is needed, which may explain why the adherence rate was about 20% higher with FIT than with FOBT in most studies.28–30

FIT has a sensitivity of 69% to 86% for colorectal cancer and a specificity of 92% to 95%.31 The sensitivity can be improved by lowering the threshold value for a positive test, but this is associated with a decrease in specificity. A single FIT has the same sensitivity and specificity as several samples.32

In a large retrospective US cohort study of programmatic screening with FIT, Jensen et al33 reported that 48% of 670,841 people who were offered testing actually did the test. Of the 48% who participated in the first round and remained eligible, 75% to 86% participated in subsequent rounds over 4 years. Those who had a positive result on FIT were supposed to undergo colonoscopy, but 22% did not.

The US Multi-Society Task Force on Colorectal Cancer34 suggests that FIT-based screening programs aim for a target FIT completion rate of more than 60% and a target colonoscopy completion rate of more than 80% of patients with positive FITs. These benchmarks were derived from adherence rates in international FIT screening studies in average-risk populations.35–39 (Note that the large US cohort described above33 did not meet these goals.) Ideally, every patient with a positive FIT should undergo diagnostic colonoscopy, but in reality only 50% to 83% actually do. Methods shown to improve adherence include structured screening programs with routine performance reports, provider feedback, and involvement of patient navigators.40–42

Accordingly, several aspects of stool-based testing need to be stressed with patients. Understanding that FOBT is recommended yearly is integral for optimal impact on colorectal cancer incidence and mortality rates.

Additionally, patients should be advised to undergo colonoscopy soon after a positive FIT, because delaying colonoscopy could give precancerous lesions time to progress in stage. The acceptable time between a positive FIT and colonoscopy has yet to be determined. However, a retrospective cohort study of 1.26 million screened patients with 107,000 positive FIT results demonstrated that the rates of cancer discovered on colonoscopy were similar when performed within 30 days or up to 10 months after a positive test. Detection rates increased from 3% to 4.8% at 10 months and to 7.9% at 12 months.43

In modeling studies, Meester et al44 showed the estimated lifetime risk and mortality rates from colorectal cancer and life-years gained from screening are significantly better when colonoscopy is completed within 2 weeks rather than 1 year after a positive FIT. Each additional month after 2 weeks incrementally affected these outcomes, with a 1.4% increase in cancer mortality. These data suggest that colonoscopy should be done soon after a positive FIT result and at a maximum of 10 months.43,44

Screening with FOBT is a multistep process for patients that includes receiving the test kit, collecting the sample, preparing it, returning it, undergoing colonoscopy after a positive test, and repeating in 1 year if negative. The screening program should identify patients at average risk in whom screening is appropriate, ensure delivery of the test, verify the quality of collected samples for laboratory testing against the manufacturer’s recommendations, and report results. Report of a positive FOBT result should provide recommendations for follow-up.

Though evidence clearly supports screening annually or biennially (every 2 years) with FOBT, the ideal interval for FIT is undetermined. Modeling studies utilized by the USPSTF and Multi-Society Task Force demonstrate that colonoscopy and annual FIT result in similar life-years gained, while 2 population-based screening programs have demonstrated that a 2- or 3-year interval may be equally efficacious by lowering the threshold for a positive test.38,45

Randomized controlled trials of screening colonoscopy vs annual and biennial FIT are currently under way. Cost-effectiveness analysis has shown that offering single-sample FITs at more frequent (annual) intervals performs better than multisample testing at less frequent intervals.45–47

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