Flexible Sigmoidoscopy or Fecal Occult Blood Testing for Colorectal Cancer Screening in Asymptomatic Individuals

Screening reduces colorectal cancer mortality

Benefits in NNT

Flexible sigmoidoscopy: 1 in 450 did not die from colorectal cancer
Fecal occult blood testing: 1 in 900 did not die from colorectal cancer
Flexible sigmoidoscopy: 0.22% did not die from colorectal cancer
Fecal occult blood testing: 0.11% did not die from colorectal cancer

Harms in NNT

Flexible sigmoidoscopy: 1 in 1250 experienced an adverse outcome
Fecal occult blood testing: 1 in 3300 experienced an adverse outcome
Flexible sigmoidoscopy: 0.08% experienced an adverse outcome
Fecal occult blood testing: 0.03% experienced an adverse outcome
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Efficacy Endpoints

Death from colorectal cancer, death from any cause

Harm Endpoints

Bleeding, perforation, and death resulting from follow-up colonoscopy or surgery


Colorectal cancer (CRC) continues to be the third leading cause of cancer death in the United States.1, 2 The aim of screening is to reduce mortality through early detection.2, 3

There are several methods available for colorectal cancer screening, such as stool-based test (guaiac fecal occult blood test and fecal immunochemical test), endoscopic methods (sigmoidoscopy and colonoscopy), and imaging methods with computed tomographic colonography among other new techniques.4 Fecal occult blood testing (FOBT) and flexible sigmoidoscopy has not been directly compared to determine the superior screening modality, if present.1

The Cochrane meta-analysis cited here assessed the effectiveness of FOBT and flexible sigmoidoscopy as a colorectal cancer (CRC) screening modality in an asymptomatic population.1 The primary outcome measured was colorectal cancer mortality.1 The meta-analysis included 9 randomized controlled trials (RCTs; 338,467 participants in the screening group and 405,919 in the control group). Flexible sigmoidoscopy as a screening modality was compared to no screening in 5 RCTs (165,733 of 414,744 participants); and guaiac-based FOBT was compared to no screening in 4 RCTs (172,734 of 329,642 participants).1 Colorectal cancer mortality was lower with flexible sigmoidoscopy (Relative risk [RR]:0.72, 95% Cl 0.65 to 0.79; Absolute Risk Difference [ARD]: 0.22%; NNT:450) and FOBT (RR 0.86, 95% Cl 0.80 to 0.92; ARD: 0.11%; NNT:900) when compared with no screening.1 In other words, one would need to screen 500 patients with flexible sigmoidoscopy to prevent 1 death resulting from colorectal cancer; whereas, one would need to screen 900 patients with FOBT to prevent 1 colorectal cancer death.1 The latter can be explained with increased follow-up colonoscopies for positive FOBT results.1 Despite this fact, neither of these modalities reduce all-cause mortality (7 trials).1

Major complications (e.g. bleeding, perforation, or death) within 30 days of screening, follow-up colonoscopy or surgery occurred in 0.03% and 0.08% of participants in the FOBT and flexible sigmoidoscopy trials, respectively.1


Despite evidence of flexible sigmoidoscopy and FOBT reducing colorectal cancer mortality, there are caveats to consider.1 Unfortunately, the meta-analysis does not provide a clear answer regarding a superior screening modality.1 In the absence of direct evidence, it cannot be concluded whether the net benefit or harm of one modality is greater than the other.1 The decision to choose one test over another should be a shared decision made by the patient and the physician. In a patient with no family history of colon cancer, FOBT as the less invasive approach could be the first logical choice. The U.S. Preventive Services Task Force guidelines recommend colon cancer screening for adults between the ages of 50-75.5

In conclusion, this meta-analysis provides high quality evidence that flexible sigmoidoscopy and fecal occult blood testing both reduce the risk of death from colorectal cancer.1 Therefore, we assign a color recommendation of Green (Benefits>Harm) to both screening tests.

The original manuscript was published in Medicine by the Numbers, American Family Physician as part of the partnership between TheNNT.com and AFP.


Karissa A. Lambert MD; Ahmed Hamed MD; Amira Hamed MD


March 29, 2019