Fecal Occult Blood Screening in the Minnesota Study: Role of Chance Detection of Lesions

Reviewers: Leonard A. Farber, M.D. and John Chang, M.D.
Source: Journal of the National Cancer Institute 1997;89:1423-8

In the past, randomized trials have found that the mortality from colorectal cancer is reduced by periodic fecal occult blood testing. In the Minnesota Colon Cancer Control Study it was found that after 13 complete follow-up years annual fecal occult blood testing reducedmortality from colorectal cancer by at least 33.4%. A large part of this reduction may have been from the cancer bleeding itself or due to chance detection of cancers by colonoscopies. This reported study was performed to determine how large the role of chance was in the 33.4% mortality reduction.

The authors utilized a mathematical model developed by Lang and Ransohoff to estimate the proportion of mortality attainable by chance alone. The model required the application of five parameters: (1) duration of follow-up, (2) rate of compliance with fecal occult blood testing, (3) rate of compliance with colonoscopy, (4) positivity rate, and (5) efficacy of colonoscopy in reducing colorectal cancer mortality. A total of 46,551 men and women aged 50-80 were randomly assigned to receive either occult blood (Hemoccult) screening annually, once every 2 years, or no screening. Randomization occurred between 1975 and 1978 and results were analyzed after 13 complete follow-up years.

Applying the Lang and Ransohoff model to the values actually observed in the Minnesota study, the authors found that chance detection was responsible for 16%-25% of the reduction in colorectal cancer deaths effected by fecal occult blood testing. The remainder, i.e., 75%-84%, was due to sensitive detection. They therefore concluded that chance played only a minor role in the detection ofcolorectal cancers by fecal occult blood testing, whereas annual fecal occult blood testing screening was the predominant factor in the 33.4% reduction in colorectal cancer deaths.