A TEST that can prove a woman has been raped even if no sperm are found
should be widely adopted, the first large-scale study suggests.
After the trauma of rape, women who report it have to undergo a medical
examination to look for sperm. But the test can fail, and as a result the
woman鈥檚 claim to have been raped may not be believed. Now a French study has
confirmed that a back-up test can often detect the male Y chromosome even if no
sperm are found.
The standard test, in which technicians use microscopes to look for sperm in
vaginal, oral or anal swabs, is very sensitive. But it can fail if the swabs are
taken more than two days after the rape, if the assailant has a low sperm count
or doesn鈥檛 ejaculate, or if the woman is menstruating or using spermicide. It
can also fail if the rape involved oral or anal sex, because salivary and
bacterial enzymes can rapidly destroy sperm.
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But an attack by a man will usually leave traces of skin cells, which will
carry the telltale Y chromosome. These chromosomes can be detected even if
there鈥檚 only one male cell for every 5000 female cells.
Philippe de Mazancourt, a forensic biologist at the Raymond Poincar茅
Hospital in Garches, France, and his colleagues used this test to examine 79
women who said they had been raped. The researchers found fragments of Y
chromosome in nearly 30 per cent of the cases for which standard sperm tests
were negative.
What鈥檚 more, they detected the Y chromosome in a third of the cases in which
the women were examined more than two days after being raped, and in two cases
nearly eight days later. 鈥淚 would recommend the test when the sperm test is
negative and when you have some indication that there was a rape,鈥� says
Mazancourt. The results will appear in Forensic Science International.
鈥淭his is a very important paper,鈥� says Howard Baum, deputy director of the
forensic biology laboratory at the medical examiner鈥檚 office in New York City.
鈥淚t is the first systematic study of the detection of Y-chromosome DNA collected
many hours after intercourse.鈥� While several countries have started using the
test, it鈥檚 not yet in widespread use.
The method has already helped convict rapists, but it does have limitations,
however. Y chromosome tests cannot uniquely identify an individual suspect
because many men share the same set of markers. 鈥淭hey鈥檒l point more to a group
than to a specific individual,鈥� says Baum. If the markers found don鈥檛 match
those of a suspect, of course, he is definitely innocent. 鈥淓xclusion is 100 per
cent,鈥� says Baum.