Prostate specific antigen (PSA) is an enzyme produced by the glandular cells of the prostate. More PSA is released when the prostate gland is enlarged, and levels of PSA in the blood are elevated in prostate cancer. PSA testing is used to identify the early stages of prostate cancer. The PSA level is also a useful reference point in treatment with high intensity focused ultrasound, allowing the doctor to see if the treatment is working.