Prostate-specific antigen (PSA), also called gamma seminoprotein or kallikrein-3, is a glycoprotein secreted by the epithelial cells of the prostate gland. It is contained in the seminal fluid, ensuring its fluidity and the free movement of spermatozoa in it. It is also believed to liquefy the mucus of the uterine lining and allow sperm to enter the uterus. Prostate-specific antigen is also present in known amounts in the blood of men. An increase in its content in it is an indicator of prostate problems, in some cases cancer. Clinical practices regarding prostate-specific antigen screening vary from country to country. In the USA and Bulgaria, for example, the recommendations of most specialists are to start screening after the age of 50. Apart from screening, prostate specific antigen levels are also considered as a prognostic criterion in the presence of localized prostate cancer, i.e. a tumor that is confined to the prostate. The other two prognostic criteria are the Gleason score – the degree of degeneration of the cancer cells, and the stage of the cancer at the time of diagnosis. Depending on the values ​​of these three criteria, the risk of premature death is divided into: low – PSA < 10 ng/ml (nanograms per milliliter), Gleason score ≤ 6 and cancer stage ≤ II A; medium – PSA between 10 and 20 ng/ml, Gleason score 7 or cancer stage ≤ II B/C; high – PSA > 20 ng/ml, Gleason score ≥ 8 or cancer stage ≥ III. NEWS_MORE_BOX Prostate-specific antigen levels are monitored periodically after initiation of treatment. If the patient has undergone a successful radical prostatectomy, which has eliminated all cancer cells from the body, levels of prostate-specific antigen in the blood become undetectable for several weeks. A possible increase of 0.2 ng/ml above the limit of detection is considered a sign of recurrence, although in rare cases this may also be due to residual benign tissue. After radiation therapy, it is possible for prostate-specific antigen levels to remain persistently detectable, even if treatment has been successful. This makes it more difficult to determine the relationship between prostate-specific antigen levels and treatment success with radiotherapy. Levels may fall gradually for up to several years after radiation treatment, after which they may become undetectable. Their rise by 2 ng/ml above this limit is taken as a sign of relapse. The interpretation of the results of the measurement of the prostate specific antigen, however, should be done cautiously and together with the results of other diagnostic methods, since prostate neoplasias are possible in which its level does not increase significantly. Also, its levels can increase in prostatitis, benign prostatic hyperplasia (benign enlargement of the prostate), etc. Apart from clinical andrology, prostate specific antigen also has applications in criminology – its presence proves the presence of ejaculate.Its emission in semen is independent of that of spermatozoa, so it is detected in it even after vasectomy or congenital sterility.