Prostate-specific antigen, or PSA, is a protein produced by normal, as well as malignant, cells of the prostate gland. The PSA test measures the level of PSA in a man's blood.
PSA, a protein produced by prostate gland cells, circulates through the body in two ways: either bound to other proteins or on its own. PSA traveling alone is called free PSA. The free-PSA test measures the percentage of unbound PSA.