A positive result on a cancer screening test means you have cancer, right? Not necessarily.
The incidence of inaccurate screening results is more prevalent than you might realize. Before your next cancer screening, here's what you need to know about one type of result: the false positive.
Cancer screening is the process of looking for potential cancer when no symptoms are present. Mammograms for breast cancer, the PSA test for prostate cancer, and colonoscopies for colon and rectal cancers are the three most widely used screening tests. Screening detects real, potentially serious cancers. It also finds harmless cancers, those that would not cause death or symptoms, or would not have been clinically identified during a person's remaining lifetime.
Unfortunately, current screening techniques can only detect the presence of something abnormal. Since physicians cannot yet tell which cancers are harmful and which are harmless, they treat all of them.
The incidence of false positive results is high. A false positive means the test produces a positive result when there is no cancer present. Physicians usually recommend additional testing, such as a biopsy, and even unnecessary treatment, after a patient's screening result is positive. All medical procedures come with risks and can cause complications.
So what do false positives mean in terms of real people and real numbers? Here are examples from a few studies to illustrate.
- If 2,000 women receive mammograms for 10 years, one woman's life will be prolonged, 10 healthy women will be treated unnecessarily for breast cancer as a result of a false positive screening result, and 200 women will undergo additional testing and procedures to confirm they don't have cancer despite a positive screening result.
- Patients who underwent low-dose computed tomography (CT) screening for lung cancer had a 21 percent likelihood of a false positive after the first screening, and a 33 percent likelihood after the second.
- When 68,000 individuals underwent a long-term screening program for multiple types of cancer (each person had a total of 14 tests), men and women had a 60 and 50 percent cumulative risk, respectively, of a false positive result, and 29 percent of men and 22 percent of women had an invasive diagnostic procedure as a result of a false positive.
Until we have better cancer screening tools, false positives (and false negatives) are a reality. In the meantime, the most important step you can take is to clearly understand the benefits, risks, and limitations of each cancer screening test you consider. Armed with knowledge, you can discuss cancer screening with your physician and then make an informed decision.
Sources:
National Cancer Institute. "What Is Cancer Screening?" Web. 12 January. 2010.
http://www.cancer.gov/cancertopics/pdq/screening/overview
Baker, Stuart G., Erwin, Diane, and Kramer, Barnett S. "Estimating the cumulative risk of false positive cancer screenings." BMC Medical Research Methodology 3:11 (2003):11doi:10.1186/1471-2288-3-11. BioMed Central. Web. 3 July 2003. http://www.biomedcentral.com/1471-2288/3/11
Markman, Maury, MD. "Balancing Benefits of Early Cancer Detection vs Risk for False Positive Results." Medscape Hematology-Oncology. Web. 3 June 2009. http://www.medscape.com/viewarticle/703660
Nelson, Roxanne. "ASCO 2009: Low-Dose CT Screening for Lung Cancer Produces High Rate of False Positives."
Medscape Medical News. Web. 4 June 2009. http://www.medscape.com/viewarticle/703909
Barclay, Laurie MD. "Lung Cancer Screening Tests May Yield False-Positive Results." Medscape Medical News. Web. 20 April 2010. http://www.medscape.com/viewarticle/720509
National Cancer Institute. "Cancer Screening." Web. 5 March 2010. http://www.cancer.gov/cancertopics/pdq/screening/overview/healthprofessional
Jorgnsen, Karsten Juhl, and Gotzsche, Peter C. "Overdiagnosis in publicly organised mammography screening programmes: systematic review of incidence trends." BMJ 339 (2009): b2587 doi:10.1136/bmj.b2587. Web.
http://www.bmj.com/cgi/reprint/339/jul09_1/b2587
Gotzsche, Peter, C., Hartling, Ole J., Nielson, Margrethe, Broderson, John, and Jorgensen, Karsten Juhl. "SCREENING FOR BREAST CANCER WITH MAMMOGRAPHY." BMJ 338 (2009): b86. Web. January 2008. http://www.bmj.com/cgi/content/full/338/jan27_2/b86
Napoli, Maryann. "Make an Informed Decision about Mammography." Centers for Medical Consumers. Web. 1 March 2009. http://medicalconsumers.org/2009/03/01/make-an-informed-decision-about-mammography/
