How Common Are False Positive Tests: Prostate Specific Antiagent PSA

Now let's consider the blood test for prostate cancer, the prostate specific antigen, or PSA. In September 1993, during "Prostate Cancer Awareness Week," a major effort was made to recruit volunteers for testing. Of the more than 30,000 men over age 50 tested in 148 centers, about 3,000 had an abnormally elevated level of PSA; that is, about 10 percent tested positively. Somewhere between a quarter and a third of these men were found to have prostate cancer, making the false positive rate about 7 percent. Others have reported false positive rates in the range of 7 to 11 percent. The most common cause of a false positive prostate specific antigen is an enlargement of the prostate gland. Because the prostate tends to enlarge with age, prostate specific antigens tend to rise with age. Therefore, false positives are less common in younger men and more common in older men.

The false positive rate for subsequent prostate specific antigen testing depends on whether the initial result was normal or not. If the men with abnormal prostate specific antigens are excluded, the false positive rate for a repeat test drops to around 2 percent. In other words, if your test was normal in the past, you are much less likely to have a falsely abnormal test in the future (which in turn means that an abnormal result in subsequent testing is more likely to represent a cancer). However, if your prostate specific antigen was abnormally elevated once, it is very likely to remain abnormal in the future-even if you never have prostate cancer.