Colorectal cancer screening: Choosing the right test

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Compared with stool-based screening, colonoscopy has advantages, including a 10-year screening interval if bowel preparation is adequate and the examination shows no neoplasia, the ability to inspect the entire colon, and the ability to diagnose and treat lesions in the same session.

Screening colonoscopy visualizes the entire colon in more than 98% of cases, although it requires adequate bowel preparation for maximal polyp detection. It can be done safely with or without sedation.48

While there are no available randomized controlled trial data on the impact of screening colonoscopy on cancer incidence or mortality, extensive case-control and cohort studies consistently show that screening colonoscopy reduces cancer incidence and mortality rates.49–54 A US Veterans Administration study of more than 32,000 patients reported a 50% reduction in overall colorectal cancer mortality.55 In a microsimulation modeling study that assumed 100% adherence, colonoscopy every 10 years and annual FIT in individuals ages 50 to 75 provided similar life-years gained per 1,000 people screened (270 for colonoscopy, 244 for FIT).56

Quality indicators for colonoscopy

Well-established metrics for maximizing the effectiveness and quality of colonoscopy have been established (Table 2). The most important include the mucosa inspection time (withdrawal time) and adenoma detection rate.57 Withdrawal time is directly correlated with adenoma detection, and a 6-minute minimum withdrawal time is recommended in screening colonoscopy examinations of patients at average risk when no polyps are found.58 The adenoma detection rate is the strongest evidence-based metric, as each 1% increase in the adenoma detection rate over 19% is associated with a 3% decrease in the risk of colorectal cancer and a 5% decrease in death rate.59 The average-risk screening adenoma detection rate differs based on sex: the rate is greater than 20% for women and greater than 30% for men.

Complications from screening, diagnostic, or therapeutic colonoscopy are infrequent but include perforation (4/10,000) and significant intestinal bleeding (8/10,000).56–62

Patients with a first-degree relative under age 60 with advanced adenomas or colorectal cancer are considered at high risk and should begin screening colonoscopy at age 40, with repeat colonoscopy at 5-year intervals, given a trend toward advanced neoplasia detection compared with FIT.63

Guidelines recently published by the Canadian Association of Gastroenterology and endorsed by the American Gastroenterological Association also support starting screening in high-risk individuals at age 40, with a surveillance interval of 5 to 10 years based on the number of first-degree relatives with colorectal cancer or adenomas.64 Consensus statements were based on retrospective cohort, prospective case-controlled, and cross-sectional studies comparing the risk of colorectal cancer in individuals with a family history against those without a family history.

Randomized clinical trials comparing colonoscopy and FIT are under way. Interim analysis of a European trial in which asymptomatic adults ages 50 to 69 were randomized to 1-time colonoscopy (26,703 patients) vs FIT every 2 years (26,599 patients) found significantly higher participation rates in the FIT arm (34.2% vs 24.6%) but higher rates of nonadvanced adenomas (4.2% vs 0.4%) and advanced neoplasia (1.9% vs 0.9%) in the colonoscopy arm.65 Cancer was detected in 0.1% in each arm. These findings correlate with those of another study showing higher participation with FIT but higher advanced neoplasia detection rates with colonoscopy.66

Detection of precursor lesions is vital, as removing neoplasms is the main strategy to reduce colorectal cancer incidence. Accordingly, the advantage of colonoscopy was illustrated by a study that determined that 53 patients would need to undergo screening colonoscopy to detect 1 advanced adenoma or cancerous lesion, compared with 264 for FIT.67


The American Cancer Society recently provided a qualified recommendation to start colorectal cancer screening in all individuals at age 45 rather than 50.9 This recommendation was based on modeling studies demonstrating that starting screening at age 45 with colonoscopy every 10 years resulted in 25 life-years gained at the cost of 610 colonoscopies per 1,000 individuals. Alternative strategies included FIT, which resulted in an additional 26 life-years gained per 1,000 individuals screened, flexible sigmoidoscopy (23 life-years gained), and computed tomographic colonoscopy (22 life-years gained).

Rates of colorectal cancer are rising in adults under age 50, and 10,000 new cases are anticipated this year.2,3 Currently, 22 million US adults are between the ages of 45 and 50. The system and support needed to perform screening in all adults over age 45 and a lack of direct evidence to support its benefits in the young population need to be considered before widespread acceptance of the American Cancer Society recommendations. However, if screening is considered, FIT with or without sigmoidoscopy may be appropriate, given that most cancers diagnosed in individuals under age 50 are left-sided.4,5

Screening has not been proven to reduce all-cause mortality. Randomized controlled trials of FOBT and observational studies of colonoscopy show that screening reduces cancer incidence and mortality. Until the currently ongoing randomized controlled trials comparing colonoscopy with FIT are completed, their comparative impact on colorectal cancer end points is unknown.


FIT and colonoscopy are the most prevalent screening methods in the United States. Careful attention should be given to offer the screening option the patient is most likely to complete, as adherence is key to the benefit from colorectal cancer screening.

The National Colorectal Cancer Roundtable (, established in 1997 by the American Cancer Society and the US Centers for Disease Control and Prevention, is a national coalition of public and private organizations dedicated to reducing colorectal cancer incidence and mortality. The Roundtable waged a national campaign to achieve a colorectal cancer screening rate of 80% in eligible adults by 2018, a goal that was not met. Still, the potential for a substantial impact is a compelling reason to endorse adherence to colorectal cancer screening. The Roundtable provides many resources for physicians to enhance screening in their practice.

The United States has seen a steady decline in colorectal cancer incidence and mortality, mainly as a result of screening. Colorectal cancer is preventable with ensuring patients’ adherence to screening. Screening rates have been shown to increase with patient-provider dialogue and with selection of a screening program the patient prefers and is most likely to complete.

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