How long can semen in stains be detected?
For years, but the long answer is: acid phosphatase (AP) can be detected in semen which has been discharged up to 17 hours after intercourse, and prostate specific antigen (PSA) for up to 36 hours. Once semen has been discharged, then its chemical components become stable for years. The explanation for this is that both AP and PSA are proteins, and must have a certain three-dimensional configuration to work. The vagina presents a hostile chemical environment however, and rapidly denatures seminal proteins in a fashion similar to egg whites when they are cooked. After discharge, the residual non-denatured proteins are no longer exposed to a hostile environment, and there is virtually no time limit for their detection. PSA actually has been detected in semen stains over 30 years old. Items tested more closely to the time of suspected intercourse give a more strongly positive test.
