Evidence of Harms

Harms are associated with the various modalities used to screen for colorectal cancer (CRC).

Fecal Occult Blood Testing (FOBT)

A systematic review done through the Cochrane Collaboration examined all CRC screening randomized trials that involved FOBT on more than one occasion. The trials reported a low positive predictive value for the FOBT, suggesting that more than 80% of all positive tests were false-positives. [1] A positive test can lead to further diagnostic procedures that include colonoscopy or double-contrast barium enema plus flexible sigmoidoscopy.


Sigmoidoscopy can be an uncomfortable or painful procedure. Women may have more pain during the procedure, which may discourage them from returning for future screening sigmoidoscopies. Sigmoidoscopy can also cause perforation and bleeding, although this is rare.


Clinically significant complications requiring medical intervention are rare but can include the following: perforations, bleeding, cardiovascular events, and other adverse events. The rate of complications may increase among older persons. [2]

Computed Tomographic (CT) Colonography

Extracolonic abnormalities are common in CT colonography. Fifteen percent of patients in an Australian series of 100 patients, referred for colonography because of symptoms or family history, were found to have extracolonic findings, and 11% of the patients needed further medical workups for renal, splenic, uterine, liver, and gallbladder abnormalities. [3] Other areas of extracolonic findings include the chest, ovary, and pancreas. In another study, 59% of 111 symptomatic patients referred for clinical colonoscopy in a Swedish hospital between June 1998 and September 1999 were found to have moderate or major extracolonic conditions on CT colonography. CT colonography was performed immediately prior to colonoscopy, and these findings required further evaluation. It is unstated to what extent the follow-up of these incidental findings benefited patients. [4]

Sixty-nine percent of 681 asymptomatic patients in Minnesota had extracolonic findings, of which 10% were considered to be highly important by the investigators, requiring further medical workup. Suspected abnormalities involved kidney (34), chest (22), liver (8), ovary (6), renal or splenic arteries (4), retroperitoneum (3), and pancreas (1). [5]


1. Hewitson P, Glasziou P, Irwig L, et al.: Screening for colorectal cancer using the faecal occult blood test, Hemoccult. Cochrane Database Syst Rev (1): CD001216, 2007. [PUBMED Abstract]

2. Warren JL, Klabunde CN, Mariotto AB, et al.: Adverse events after outpatient colonoscopy in the Medicare population. Ann Intern Med 150 (12): 849-57, W152, 2009.[PUBMED Abstract]

3. Edwards JT, Wood CJ, Mendelson RM, et al.: Extracolonic findings at virtual colonoscopy: implications for screening programs. Am J Gastroenterol 96 (10): 3009-12, 2001.[PUBMED Abstract]

4. Hellström M, Svensson MH, Lasson A: Extracolonic and incidental findings on CT colonography (virtual colonoscopy). AJR Am J Roentgenol 182 (3): 631-8, 2004.[PUBMED Abstract]

5. Gluecker TM, Johnson CD, Wilson LA, et al.: Extracolonic findings at CT colonography: evaluation of prevalence and cost in a screening population. Gastroenterology 124 (4): 911-6, 2003.[PUBMED Abstract]

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