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's 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 does not 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.

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's only one male cell for every 5000 female cells.

Eight days later

Philippe de Mazancourt, a forensic biologist at the Raymond Poincaré 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's 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. "I would recommend the test when the sperm test is negative and when you have some indication that there was a rape," says Mazancourt. The results will appear in Forensic Science International.

"This is a very important paper," says Howard Baum, deputy director of the forensic biology laboratory at the medical examiner's office in New York City. "It is the first systematic study of the detection of Y-chromosome DNA collected many hours after intercourse." While several countries have started using the test, it's 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. "They'll point more to a group than to a specific individual," says Baum. If the markers found don't match those of a suspect, of course, he is definitely innocent. "Exclusion is 100 per cent," says Baum.