An ENZYME produced by glandular tissue in the PROSTATE GLAND. When the gland enlarges (see PROSTATE GLAND, DISEASES OF), greater amounts of PSA are secreted, raising the concentration of the enzyme in the blood. This is especially so in cancer of the prostate, and testing the level of PSA is an indicator that the disease may be present. Measuring PSA is a standard part of aftercare of patients who have been treated for prostate cancer as it is a guide to whether the disease might have returned. However controversy remains about the use of PSA as a screening test (see PREVENTIVE MEDICINE – SCREENING). Its proponents claim that population screening would reduce deaths from prostate cancer by enabling earlier diagnosis; its opponents point to epidemiological evidence, for example from Australia where screening was introduced, which suggests that although it increases diagnosis it does not reduce the death rate. The suggested reason is that all the test results in is to bring to light many cases that would never have needed treatment as the cancers would have remained small and confined to the prostate. Further, if the level of PSA is very high when the person is screened, the disease is already advanced; where the result is equivocal it is uncertain whether the benefits of treatment outweigh the risks.