A PSA test is a blood test used to measure the level of PSA in the blood.  Many have heard the 3 letter abbreviation, especially if you are a man over 50, but what exactly is a PSA?  PSA is the abbreviation for Prostate-Specific Antigen, which is a protein produced by the cells of the prostate gland.  The PSA is often called a biological or tumor marker because it can be used to detect disease.  The disease in this case is prostate cancer.  Therefore the PSA test measures the level of PSA in the blood, like we mentioned above, in order to screen for prostate cancer.