PSA is prostate specific antigen. It's a molecule specific to the cells that make up the male prostate gland.
Prostate cancer disrupts prostate cells and causes the release of PSA into the blood. A simple blood test can measure blood levels of PSA.
The PSA test originally was developed as a way to tell whether prostate cancer was coming back in men already treated for prostate cancer.
But the higher a man's PSA level, the more likely it is he has prostate cancer. Since PSA levels begin to rise early in the course of prostate cancer, the PSA test can detect prostate cancer before it becomes dangerous.
With the PSA test in hand, doctors began giving the test to healthy men with no symptoms of prostate cancer. By 1991, routine PSA screening became widespread in the U.S. -- a year before the start of the first large clinical trial designed to see if PSA screening actually saved lives.
Do high PSA levels always mean prostate cancer?
No. PSA blood levels go up for other reasons besides cancer.
Other things that affect the prostate -- a digital rectal exam, urine retention, or even ejaculation -- can also cause a rise in PSA.
In the U.S., doctors usually get suspicious when a PSA level is 4.0 ng/mL. But studies show that many men with levels well above 4.0 ng/mL do not have cancer. And many men with prostate cancer have PSA scores well below this level.
There are a lot of ways to tweak the PSA test to try to get better results. For example, PSA comes in two forms, free and attached. BPH and other non-cancer conditions tend to increase the free form, while cancer tends to produce more of the attached form. Measuring free vs. attached PSA may help determine whether a prostate biopsy really is necessary, but more research is needed.
Similarly, measuring how fast PSA goes up over time (PSA velocity) or the relationship between PSA and the size of the prostate (PSA density) may help identify men at higher risk of cancer. But without more research, these measures remain controversial.