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.