The PSA test may be used to screen both asymptomatic and symptomatic men for prostate cancer. PSA is a protein produced primarily by cells in the prostate and most of the PSA is released into semen, but small amounts of it are also released into the blood. PSA exists in two forms in the blood: free (not bound) and complexed (cPSA, bound to other proteins). Lab tests can measure free PSA or total PSA (bound plus unbound).