Semen Detection - "Now it's there, now it's not"

Article taken from Issue 2 of the Forensic Access Newsletter "Benchmark"

Contrary to popular belief, identifying trace amounts of semen on vaginal swabs is not always especially straightforward.

Forensic scientists tend to use a combination of chemical screening and microscopical tests for the purpose. The chemical screening test is applied first, and detects the presence of a substance found at particularly high levels in human semen known as acid phosphatase - or AP for short. The difficulty here is that vaginal material itself can contain some AP activity and, although this usually gives a slightly different reaction in the test, it can on occasion be confused with what might be expected from semen in trace amounts.

The microscopical test involves treating some material extracted from swabs with a combination of biological stains which react in a characteristic way with different parts of the sperm, and then searching for sperm heads under the microscope (sperm tails are especially fragile and tend to become detached from the more robust heads at the drop of a hat).

Difficulties here can involve identifying what is, and is not a sperm where there are very few about and, for one reason or another, they may be of poor quality and not so sharply defined or so characteristically stained as might otherwise be the case.

A case in point concerned a man who was charged with indecently assaulting his young female relative. Support for the charge was apparently provided in the form of two sperm on an intimate swab taken from the alleged victim. By any standards this represented a truly tiny amount of semen - a normal ejaculate would be expected to contain of the order of several hundred million sperm.

Attempts had been made to obtain a DNA profile from the swab and which might be associated with the semen. These failed, but this was not unexpected in view of the very little material apparently there.

Badly contaminated sample
In any event, we were instructed to visit the laboratory concerned primarily to check out the DNA test failure, but during the course of this we needed to confirm the identification of semen on the swab. We immediately rejected one of the supposed sperm and we were extremely doubtful about the other one. The sample was badly contaminated with fungal spores and these were staining up rather as one might expect deteriorating sperm to do.

In an effort to resolve the matter, we made further microscope preparations from the swab and invited the police scientist to do the same, and to check the original ones and get colleagues also to pass an opinion on them.

At the end of the day, all agreed that it was entirely unsafe to suggest that there were any sperm on the swab at all. In a subsequent prosecution statement, the evidence was withdrawn. The prosecution case collapsed. This demonstrates how careful one has to be about scientific evidence - even of apparently the most basic kind.