The PSA test is a blood test that measures the amount of prostate specific antigen (PSA) in your blood. Prostate specific antigen (PSA) is produced by healthy cells in the prostate, so it’s normal to have a small amount of PSA in your blood. The amount rises as you get older, because your prostate gets bigger (enlarged prostate), but it can also rise in the presence of prostate cancer, prostatitis, vigorous exercise and ejaculation (you might be asked not to do any vigorous exercise and avoid any sexual activity in the 48 hours before a PSA test).