The problem w/ PSA is that the test is a snapshot of a function which fluctuates widely over time, much as blood sugar and blood pressure do. In signal processing, we would refer to PSA test results as seriously undersampled. That's why it is so important to have the same conditions over the week leading up to each PSA reading. John V. is correct that the only thing that really matters is rate of change, rather than the absolute level itself, which varies widely among healthy men. PSA is a flawed test with an absurdly high false positive rate, but it is unfortunately the best we have with today's technology. I fully expect it to be obsolete and replaced by a more reliable test by the time my sons are old enough to worry about prostate cancer.