Asked by Charles, Minnesota
What is the percentage of false positives from the PSA test?
Dr. Otis Brawley
Chief Medical Officer,
American Cancer Society
Prostate specific antigen is a blood test that is approved by the Food and Drug Administration for following the response to treatment and progression of diagnosed prostate cancer. It is also approved for diagnostic use in men who have symptoms that might be prostate cancer. Ironically, it is most commonly used to screen asymptomatic men for prostate cancer. It is not FDA-approved for screening. Its use as a screening test has been the subject of some controversy for nearly 20 years.
Part of the controversy was no study had shown that prostate cancer screening saved lives until last year. It clearly diagnosed a lot of cancer, and many experts thought it saved lives, but this was not certain. Two long-term clinical trials were published in the spring of 2009. One showed that screening does reduce the risk of prostate cancer death by 20 percent after about 10 years of follow-up. The second study did not verify this finding. Both studies did show that screening diagnosed a lot of men, who even though they had prostate cancer, it was a disease that was not a threat to their life. Without screening, these men would never have been bothered by the disease. The study that showed a lifesaving benefit, also showed that 48 men had to be treated for prostate cancer to save one life. Treatment for prostate cancer can have significant side effects, including include urinary incontinence, erectile dysfunction and even death.
In response to these trials, the American Urological Association, the European Association of Urology and the American Cancer Society (for which I work) have all issued statements recommending that men be informed of and understand the complexities of screening which include the known risks and the potential for benefit before they undertake screening. Men really should have a good conversation with a physician who understands the issues and then make a decision for themselves about screening. Most professional organizations discourage mass screening as the necessary teaching and understanding cannot be assured.
Some of the risks of prostate screening include the false positive rate. The false positive rate is the number of men with an abnormal PSA, who do not have cancer. For some years a cutoff of 4.0 ng/mL was used. A prostate biopsy was done on men with a PSA greater than 4.0. In a large case series 7.6 percent of men over age 50 had a PSA over 4.0 and 25 percent of men with a PSA over 4.0 had prostate cancer.
Recent studies have shown that a large number of men have prostate cancer even though they have a PSA less than four. Indeed, in one study in which men with PSAs less than four were biopsied anyway, 15 percent had prostate cancer. Even when we consider a normal PSA as being 2.5 ng/mL, a significant proportion of cancers will be missed. We desperately need a better test to find the disease.
In a large long-term study of "normal risk" American men over 55 years of age, 28 percent were diagnosed with prostate cancer. Half were diagnosed because of an abnormal screening test over a seven-year period. Half were diagnosed by biopsy after having a normal screening test annually for seven years. An abnormal screening test in this study was considered a PSA over 4.0, a rise in PSA of 1.0 ng/mL in one year, or an abnormal digital examination of the prostate. In addition to the PSA level at a blood draw, change in PSA over time and digital examination of the prostate are commonly used in prostate screening.
This study demonstrates that we also need a test that can tell us the cancers that need to be treated because they are a risk to the man's health and those that need to be observed. In the study in which 28 percent of men were diagnosed with prostate cancer (half due to an abnormal screen). Scientists estimate that only 3 percent of these men would have died of prostate cancer. There is not a good test to figure out who needs to be cured and who did not need to be cured.
There are several online sources for more information and decision tools to help men decide if they want to be screened for prostate cancer. In addition to those linked above, Informed Health Choice can help you decide whether to have a PSA test.
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