Prostate-specific antigen.

Prostate-specific antigen (PSA) also known as gamma-seminoprotein or kallikrein-3 (KLK3) is a glycoprotein that in humans is encoded by the KLK3 gene. KLK3 is a member of the kallikrein-related peptidase family that secreted by the epithelial cells of the prostate gland. PSA is produced for the ejaculate where it liquifies the semen in the seminal coagulum and allows sperm to swim freely. It is also believed to be instrumental in dissolving the cervical mucous, allowing the entry of sperm.The discovery of prostate-specific antigen (PSA) is beset with controversy; as PSA is present in prostatic tissue and semen, it was independently discovered and given different names, thus adding to the controversy,PSA was first identified by researchers attempting to find a substance in seminal fluid that would aid in the investigation of rape cases. PSA is now used to indicate the presence of semen in forensic serology. The semen of adult males has PSA levels far in excess of those found in other tissues; therefore, a high level of PSA found in a sample is an indicator that semen may be present. Because PSA is a biomarker that is expressed independently of spermatozoa, it remains useful in identifying semen from vasectomized and azoospermic males.
PSA can also be found at low levels in other body fluids, such as urine and breast milk, thus setting a high minimum threshold of interpretation to rule out false positive results and conclusively state that semen is present. While traditional tests such as crossover electrophoresis have a sufficiently low sensitivity to detect only seminal PSA, newer diagnostics tests developed from clinical prostate cancer screening methods have lowered the threshold of detection down to 4 ng/mL. This level of antigen has been shown to be present in the peripheral blood of males with prostate cancer, and rarely in female urine samples and breast milk. No studies have been performed to assess the PSA levels in the tissues and secretions of pre-pubescent children. Therefore, the presence of PSA from a high sensitivity (4 ng/mL) test cannot conclusively identify the presence of semen, so care must be taken with the interpretation of such results.