10.5 - Risks from Screening

Tests have false positives and false negatives. But there are risks associated with screening also. Recently, risks of screening are being given more attention.

True positive

What would be the risk associated with an accurate diagnosis? In some situations, a person may prefer to not know their diagnosis. What if there is no effective treatment at present? What if having the disease brings certain consequences in health insurance policies or social standing? A risk of being a true positive is the “labeling effect” .

False positive

Among the negative consequences are the following:

  • Monetary loss
  • Harm from confirmatory tests
  • Anxiety
  • Fear of future tests


True negative

The negative consequences here are the needless costs and risks of the screening tests.

False negative

If these individuals have some assurance that they don't have disease, they may no longer seek health care or may disregard early symptoms. Their risks include:

  • Delayed intervention
  • Disregard of early signs or symptoms

Example: Risks Associated with Colorectal Cancer Screening

Let's take a look at an example related to colorectal cancer. Winawer et al simulated the effects of a screening program of annual fecal occult blood tests on one-thousand persons over 35 years, i.e. from age 50 until age 85. What were the consequences?

The table below provides the simulation results. Over the 35 years, 27,030 instances of FOBT-screening were performed. 2263 colonoscopies were performed as follow-up to a positive FOBT-screening test. The colonscopy test was negative for 2158 of the 2263 persons, but colonoscopy is not without potential complications! Complications can include death, perforation of the bowel, major bleeding or minor complications. Notice the incidence of these harms that came from what seemed like an innocuous screening program.

Clinical Consequences for 1000 People Entering a Program of Annual FOB T-Screening for Colorectal Cancer at Age 50 and Remaining in the Program Until 85 Years of Age Clinical Consequences

  Screening tests 27,030.0
  Diagnostic evaluations (by colonoscopy) 2,263.0
  False-positive screening tests 2,158.0
  Deaths due to colonoscopy complications) 0.5
  Bowel perforations from colonoscopy 3.0
  Major bleeding episodes from colonoscopy 7.4
  Minor complications from colonoscopy 7.7
  Death averted 13.3
  Years of life saved 123.3
  Years of life gained per person whose cancer death was prevented 9.3

Adapted from Winawer SJ, Fletcher RH, Millar L, et al. Colorectal cancer screening: clinical guidelines and rationale. Gastroenterology 1997;112:594-642.

There are also benefits to colorectal screening. Death was averted in 13 individuals, which works out to 123 years of life saved, 9.3 years per person..

 Think about it!

How would you compare 123 years of life saved compared to one person dying needlessly because they underwent this screening program? At the population level? As an individual?