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( Prostate specific antigen)
 Prostate specific antigen (PSA), also known as kallikrein III, seminin, semenogelase, ?-seminoprotein and P-30 antigen) is a 34 kD glycoprotein manufactured almost exclusively by the prostate gland; PSA is produced for the ejaculate where it liquifies the semen and allows sperm to swim freely.[1] It is also believed to be instrumental in dissolving the cervical mucous cap, allowing the entry of sperm.[2] Biochemically it is a serine protease (EC 3.4.21.77) enzyme, the gene of which is located on the nineteenth chromosome (19q13). [3] PSA is normally present in the blood at very low levels. The reference range of 0-4.0 ng/mL for the first commercial PSA test, the Hybritech Tandem-R PSA test released in February 1986, was based on a study that found 99% of 472 apparently healthy men had a total PSA level below 4 ng/mL—the upper limit of normal is much less than 4 ng/mL.[5] Increased levels of PSA may suggest the presence of prostate cancer. However, prostate cancer can also be present in the complete absence of an elevated PSA level, in which case the test result would be a false negative.[6] Obesity has been reported to reduce serum PSA levels.[7] Delayed early detection may partially explain worse outcomes in obese men with early prostate cancer.[8]
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