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Monday article #37: Sexual assault Biological Evidence: Their management and workflow.

1 in 6 American women and 1 in 33 American men had experienced attempted or completed sexual rape when they were a child or adults (Kuadli, 2021). As shocking as it sounds, it is a sad reality that continues to persist while increasing each year. Moreover, false sexual allegations exist; in some countries, they can break a person’s career, highlighting the importance of justice in a court. However, from a legal point of view, evidence plays a significant role and biological evidence is often used in such cases. So how are they managed and what is the workflow when it comes to biological forensics evidence?

In a case of a sexual assault crime, several pieces of evidence can be collected when the victim and perpetrator have come into contact. The list of biological pieces of evidence can range from saliva to vaginal fluid and even semen. There are usually two types of evidence. One is known as direct evidence which includes the victim’s own statement and eyewitness. The other one is known as Circumstantial evidence which requires investigating to look for matches between the victim or perpetrator, hence, most of the biological evidence would fall under this category. Regardless of which category of evidence is given, the sole purpose is to provide these six main uses:

  1. Align police investigation

  2. Allow a reliable source of information that can identify the perpetrator

  3. Release a suspect from false claims

  4. Be able to support or contradict a victim's, witness', or suspect's statement

  5. Give details on the crime scene

  6. Have the ability to provide proof that attests to the occurrence of the alleged event (Magalhães et al., 2015).

The most basic form of workflow for a forensic examination that involves a biological sample usually goes through a five-stage process that includes a visual examination, presumptive assay, confirmatory assay, blood typing, and DNA typing. However, it is to be noted that different types of biological samples will have slight alterations to the workflow. In this article, the primary example used to illustrate the workflow and the methods used is semen.

To visually identify the biological sample as semen, there are a few ways. Semen usually has a distinct odour; when it is dried, it appears to be pale yellow or gray. A variable wavelength light source such as Polilight can also help to visually identify the semen. Moving on to the presumptive assay, semen usually contains sperm and seminal plasma which is the main target to be tested. In seminal plasma, there is a high concentration of acid phosphatase and usually, in sexual crimes, there is an SM kit that is used to detect high concentrations of acid phosphatase in order to prove further that the biological sample is in fact semen. When the presumptive assay appears to be positive, a sperm test uses microscopic methods as a confirmatory assay. There are a number of staining methods utilised such as Baecchi, Corin-Stockis, Christmas tree (Oppitz), and hematoxylin and eosin staining. The Christmas tree is particularly useful as it gives a red colour to the head of the sperm and a green colour to the tail which makes it easily identifiable as shown in figure 1. Apart from sperm tests, serological methods involving antibodies are also used to provideroof of seminal plasma. Blood typing tests for the ABO blood type were used in the past when DNA typing was not available, in the present day, it is sometimes omitted or used to narrow down the suspects (Sakurada, Watanabe and Akutsu, 2020).

As technology advances, there has been an addition of DNA typing to provide the genetic profile of the criminal. Specifically for semen, a Y-STR(Y-chromosome testing) profiling method can be used instead of an autosomal profiling method as there are challenges when it comes to mixed biological samples of men and women or a case of a sample that has a prolonged post-coital time interval. Y-STR is able to specifically target the explicitly targets STR regions on the male Y chromosome that has been passed down from the paternal lineage. In table 1, it illustrates some of the benefits of this test (Amy Jeanguenat, 2015).

According to Hall and Ballantyne (2003), “19 Y-STR loci systems (MPI and MPII) permit a reliable high-resolution haplotype determination of the semen donor in cervicovaginal samples taken up to 48 h after intercourse”. Hence, it shows to be extremely useful in bringing justice to a sexual assault crime.

To conclude it all, sexual assault crimes continue to be an issue of the world and biological samples used as evidence are one of the main players to bring justice to the court. Therefore, there is a workflow and methods or new research to provide the most reliable evidence in order to be fair as well as provide justice to the victims.


  1. Amy Jeanguenat (2015). Y-STR Testing: Enhancing Sexual Assault and Cold Case Workflows. [online] Available at:

  2. Denis Andrei Ispan (2018). Validation of ‘Christmas tree’ staining method for microscopic observation of spermatozoa. [online] Available at: [Accessed 28 Nov 2022].

  3. Hall, A. and Ballantyne, J. (2003). Novel Y-STR typing strategies reveal the genetic profile of the semen donor in extended interval post-coital cervicovaginal samples. Forensic Science International, [online] 136(1-3), pp.58–72. doi:10.1016/s0379-0738(03)00258-5.

  4. Kuadli, J. (2021). 32 Shocking Sexual Assault Statistics for 2022. [online] Available at: ment%20from,is%20both%20disheartening%20and%20unnerving. [Accessed 31 Oct. 2022].

  5. Magalhães, T., Dinis-Oliveira, R.J., Silva, B., Corte-Real, F. and Nuno Vieira, D. (2015). Biological Evidence Management for DNA Analysis in Cases of Sexual Assault. The Scientific World Journal, [online] 2015, pp.1–11. doi:10.1155/2015/365674.

  6. Sakurada, K., Watanabe, K. and Akutsu, T. (2020). Current Methods for Body Fluid Identification Related to Sexual Crime: Focusing on Saliva, Semen, and Vaginal Fluid. Diagnostics, [online] 10(9), p.693. doi:10.3390/diagnostics10090693.


This article was prepared by Chloe Wong


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