Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 65-year-old man is evaluated during a visit to establish care. He is interested in colorectal cancer screening; however, he adamantly refuses to undergo colon preparation, and he does not want to modify his diet for screening. He has never undergone colorectal cancer screening. Medical and family histories are unremarkable. He takes no medications.

Physical examination, including vital signs, is normal.

After discussing the colon preparation process and dietary restrictions with the patient and exploring his concerns, he is steadfast in his refusal.

Which of the following is the most appropriate screening test for this patient?

A. Circulating methylated SEPT9 DNA test
B. CT colonography
C. Fecal immunochemical test
D. Sensitive guaiac-based fecal occult blood test

MKSAP Answer and Critique

The correct answer is C. Fecal immunochemical test.

The most appropriate screening test for this patient is a fecal immunochemical test (FIT). The U.S. Preventive Services Task Force (USPSTF) recommends screening for colorectal cancer in asymptomatic adults aged 50 to 75 years. For patients with average risk for colorectal cancer, several screening strategies are available, including fecal occult blood testing, direct endoscopic visualization, radiologic examination, and testing the blood for molecular markers of cancer. There is little head-to-head comparative evidence that any one recommended screening modality provides a greater benefit than the others. In addition, despite unequivocal evidence that colon cancer screening reduces mortality, an estimated one in three U.S. adults who are eligible for colon cancer screening has not been screened. Therefore, the USPSTF supports using the test that is most likely to result in completion of screening. Test selection should be guided by evidence, patient preferences, and local availability. Two fecal blood detection tests are available: a sensitive guaiac-based fecal occult blood test (gFOBT) and an FIT that uses antibodies to detect human hemoglobin. Sensitive gFOBT requires dietary restriction in order to reduce false-positive results, whereas FIT does not. The FDA has approved a third stool-based screening test that is combined with FIT and detects cancer DNA in the stool (the multitargeted stool DNA test). Mortality data for this screening strategy are not available. Because this patient would prefer not to modify his diet, FIT is the most appropriate screening option.

The plasma circulating methylated SEPT9 DNA test is an FDA-approved colorectal cancer screening test that holds promise, as blood tests may result in increased screening adherence. However, its sensitivity for detecting colorectal cancer is suboptimal at 48%, and mortality data are lacking.

Endoscopic tests include flexible sigmoidoscopy and colonoscopy. The mortality benefit of flexible sigmoidoscopy is limited to cancers of the distal bowel. Colonoscopy can visualize the entire bowel but requires colon preparation, which can be a barrier to completing the study. CT colonography is a radiologic technique that also requires colon preparation, which this patient has refused.

Major guidelines differ in their recommendations regarding screening strategy and frequency. The 2016 USPSTF guideline recommends sensitive gFOBT or FIT annually or multitargeted stool DNA testing every 3 years. Flexible sigmoidoscopy is recommended every 5 years, but if combined with FIT (or possibly gFOBT), the interval can be increased to every 10 years, the same interval recommended for colonoscopy. CT colonography can be performed every 5 years.

Key Point

• The U.S. Preventive Services Task Force recommends screening for colorectal cancer in asymptomatic adults aged 50 to 75 years; the choice of screening test should be guided by evidence, patient preferences, and local availability.

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