Living Well The American Medical Association Journal of Ethics recently discussed the dangers of doctors not understanding the math of false positive results on prostate cancer screenings...
Assume you want to perform a prostate-specific antigen (PSA) screening test on a patient who lives in a specific region of the country. You know the following information about men in this region:
- The probability that a man has prostate cancer is 1% (prevalence).
- If a man has prostate cancer, the probability that he tests positive is 90% (sensitivity).
- If a man does not have prostate cancer, the probability that he nevertheless tests positive is 9% (false-positive rate).
During the pre-screening discussion with this patient, he asks you what the chances are of having prostate cancer if the test comes back positive. What is the best answer?
- The probability that he has prostate cancer is about 81 percent.
- Out of 10 men with a positive PSA test, about 9 have prostate cancer.
- Out of 10 men with a positive PSA test, about 1 has prostate cancer.
- The probability that he has prostate cancer is about 1 percent.
The best answer is "C" – one out of every 10 men who test positive in screening actually has prostate cancer. The other nine are false alarms.
This is a great question to ask your doctor when he recommends a PSA test.
Next time you're going in for a check-up, print it out and bring it with you.
PSA tests are notorious for giving false-positive results because benign factors can cause elevated PSA levels. Inflammation, infection, recent ejaculation, and even riding a bike can increase your PSA levels.
Most healthy adults don't need a PSA test. But if your doctor recommends a PSA screening, make sure he also explains and understands the statistics and the possibility of a false positive.
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