Washing out the stains
Locating semen stains on clothing usually relies initially on a chemical screening test followed by a microscopic examination for spermatozoa (sperm cells) to confirm the presence of semen. But are the tests used sensitive enough to detect semen after an item has been washed?
The term ‘washed’ can mean many different things ranging from a quick rinse, a full wash cycle in a washing machine or soaking and then washing for example. How long the item is immersed in water, the type of detergent and temperature used will have some effect on whether semen can be detected afterwards. A scientist will need to be given as much information as possible about the type of washing used so that they can advise on the likelihood of detecting any staining and decide what tests and strategy they should employ. This is also relevant when an expert is asked to comment on whether stains are likely to be found on a washed item, and whether the laboratory testing was appropriate.
Examining washed items in sexual offence cases
Clothing and bedding are often washed prior to a rape or other sexual offence being reported to the police. The chemical test used to locate possible semen staining detects a water soluble substance in semen called acid phosphatase (AP). Washing is likely to significantly reduce, and often completely remove any AP present. However, traces of AP may remain on items given a short rinse or a wash at low temperature. When a pair of knickers has been worn after sexual intercourse, semen is likely to be deposited in the crotch area. Small numbers of sperm cells can be retained in the fabric even after washing and may be sufficient to give a DNA profile. The scientist can target areas likely to be stained for examination for the presence of sperm cells, however with large items, such as bedding, this approach is usually not practical.
Another body fluid that might be of interest in a sexual offence is saliva. Washing is likely to remove all detectable traces of saliva. The tests currently used to locate these body fluids aren’t sensitive enough to detect the traces that may remain.
Evaluation relies on contextual information
Whether semen could remain on an item after washing can become important in the context of a case. In a situation where intercourse had occurred consensually between two parties in the past, but an alleged non-consensual act of intercourse had subsequently occurred, the presence of trace amounts of sperm cells in the crotch of a pair of knickers may be the result of past acts of intercourse even though the knickers had been washed multiple times in the intervening period.
It should also be considered whether traces of semen on an item could have been transferred innocently during the washing process. A recent study by Canadian researchers demonstrated that sperm cells could be transferred from semen stained bedding to clean underwear during washing in a machine. Identifiable male DNA profiles were obtained. The same group also looked at the transfer of female vaginal material in the wash and the background levels of DNA and semen present on underwear from children. It appears to be relatively common to detect low levels of DNA from family members on a child’s underwear, but the male DNA is not necessarily attributable to semen.
Why is it vital to be able to identify the body fluid on a washed item?
Today’s DNA profiling tests are extremely sensitive and profiles can be obtained from body fluid traces which might remain after an item has been washed. In many cases, it is vital to establish from what body fluid the DNA profile originated. If a body fluid such as semen has been identified in the area sampled, a scientist can give an opinion about whether the DNA detected came from the semen. This is, however, a matter of opinion and requires consideration of the test results, DNA profiling results and case contextual information. Where only traces of body fluid remain, this is by no means straight forward and is complicated by the presence of DNA from multiple donors. One of our case studies (Rape – did the DNA come from semen?) highlights some of these issues.
Forensic Context’s experts have wide experience of interpreting the scientific results where issues may involve an item having been washed. We are always happy to advise – give us a call or email us.
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