Another test that I am seeing going by the wayside in some forensic laboratories, is the test for the presence of semen. Instead of examining an item with an Alternate Light Source looking for fluorescing stains, followed by presumptive semen testing with the Acid Phosphatase test and finally a microscopic examination for the presence of sperm cells, I now see labs – apparently in an effort to save time and streamline testing – going straight to extraction and quantitation of an apparent stain or sexual assault kit swab.
While this method of “Male Screening” can indeed tell the analyst if male DNA is present, it does nothing to inform the analyst regarding the number of sperm cells present, whether saliva may also be present in the sample, or whether any tails are present on the sperm cells. Why is this important? Because the number and quality of the sperm cells can help one to determine the relative time since intercourse (TSI) interval. While the TSI might not be important in all cases, it can be critical in some.
For instance, if a female states that she had consensual intercourse with her boyfriend on Sunday night but after an argument he raped her on Wednesday morning and she immediately sought medical attention, screening the vaginal swabs for the presence of male DNA will probably give you a positive result regardless of when the sexual intercourse occurred (Sunday night or Wednesday morning). However, if the sample were examined microscopically, then additional information could be obtained. If the microscopic examination revealed a large number of sperm cells with some or most having tails present, this would indicate a much more recent ejaculation than if there were a few sperm present without tails. A “many sperm” sample would tend to support the alleged victim’s statement while a “few sperm” sample would tend to support the suspect’s statement (assuming internal ejaculation and no condom use each time).
Similarly, if a murder victim is located – especially one who may have been a prostitute – and her vaginal swabs are positive for a low number of sperm cells, this could be more indicative of sexual intercourse a day or more before her death and therefore the person whose sperm is present in her body is not necessarily the person who killed her.