News January 25, 2022

By Forensic Access

In cases of sexual assault, biological testing and the examination of exhibits may allow scientists to recover evidence of human contact. The evidence could be cellular material deposited in the act of touching or in body fluids such as semen or saliva. DNA information can then be obtained from these samples to assist in identifying who this sample may be from.

Bodily fluid evidence can be collected from the complainant, suspects and/or the crime scene to help forensic scientists assist in determining how and when a sexual assault took place. Sexual assault cases are often complex with the detail of the competing accounts of complainant and defence crucial to any evaluation.


Detection and identification of body fluids

In the first instance, forensic biology involves the detection and identification of body fluids. Our scientists will examine an item or possible scene of crime and our first step is to try to detect any body fluids that might be present and attempt to identify what that body fluid is.


Evaluating DNA evidence and distribution of the body fluid

We can then submit a sample of that body fluid for DNA profiling to identify who the body fluid may have come from, and, where possible, statistically evaluate any potential match.

The next step is to analyse the distribution of the body fluid and determine whether we can provide any comment as to how or when it was deposited.


Explaining the results

The next stage in the process involves considering versions of events put forward by the complainant and the defendant, often referred to as the prosecution hypothesis and defence hypothesis. This is often crucial in sexual assault cases where a degree of contact or involvement may be agreed by both parties, and it is instead specific details that vary between the two accounts.

The role of the forensic scientist is to consider are the results more readily explained by the prosecution hypothesis, the defence hypothesis, both or neither? If the latter, do the results point towards an entirely new and different version of events?


Do our results lend support?

Where possible, we will consider whether our results lend support to one account over another. And if so, can we say how much? It is important to note that this is only possible in some cases depending on the accounts given and evidence found.

Our aim in every case, whether we are working as a prosecution scientist or a defence scientist, is to safely provide as much robust information as possible. Where possible we will comment on each of the areas noted above, However, this is highly dependent on the results obtained as well as the accounts given.

For example, in one instance we may be able to give robust comment as to what body fluid(s) is present, who it may have come from, how and when it was likely deposited and then use that evidence to support one version of events over another. In another case, however, it may only be possible to say that DNA was present but not comment as to what body fluid it is from, or how or when it was deposited.  



This is the body fluid most looked for in cases of sexual assault. The normal male ejaculate is between ~2.5ml – 6ml, with over 100 million spermatozoa per ml. Semen is made up of seminal fluid containing spermatozoa. However, there is also the possibility of azoospermic semen and oligospermic semen. Azoospermic semen is totally absent of spermatozoa (such as may be the case with a vasectomized male) whilst oligospermic semen has very few spermatozoa present. Pre-ejaculate is a liquid emitted from the penis prior to ejaculation. It is not semen; however, various studies have shown that pre-ejaculate can contain small amounts of spermatozoa. Therefore, it is possible that we may still detect small amounts of spermatozoa even when it is thought that ejaculation has not occurred.


Detecting semen

Detecting semen on an item is a two-stage process.

  1. Detection of acid phosphatase

The first stage of examination is to look for a component usually present at high levels in seminal fluid. This is called acid phosphatase. Acid phosphatase is water soluble, and because of this we spray the item with water. We then press white blotting paper onto the item and dampen this as well. If semen is present on the item, some acid phosphatase will solubilize in the water and leach onto the blotting paper. We then very carefully mark where the item is under the blotting paper.

We remove the blotting paper, and spray it with a chemical called AP solution. This chemical reacts with the acid phosphatase to change from an orange colour to purple.

If we get a purple colour change acid phosphatase may be present, therefore seminal fluid may be present. We then take our blotting paper, realign it over the item and mark where that reaction is.

  1. Confirming semen

The acid phosphatase test can react with other substances. For example, vaginal material can contain small amounts of acid phosphatase. Tea can also occasionally give a false positive reaction to this test, along with some deodorants and aftershaves. Therefore, our next step is to see if we can confirm the presence of semen on the item.

To do that, we will cut out and extract a small part of the fabric that’s tested positive for seminal fluid. We look at a portion of this sample under a microscope for spermatozoa. If we find spermatozoa, we can conclusively state that semen is present.

We can, as discussed above, attempt to obtain a DNA profile from the semen, evaluate any distribution to assess how and possibly when it was deposited, and evaluate our findings in light of competing prosecution and defence accounts.


Body Fluid Expertise

Forensic Access boasts a world-class team of forensic scientists operating in bespoke facilities. All forensic work is carried out to the highest quality standards which is imperative when working with body fluids.

All defence work is handled by dedicated Casework Managers who provide end-to-end support and streamlining. They can facilitate direct access to our team of experts to ensure both barristers and solicitors not only get their questions answered efficiently and effectively, but also understand the issues of their case from a scientific standpoint, giving them scope to prepare a more effective defence strategy and challenge the evidence in the correct way.

To find out more about our forensic services, call us on 01235 774780 to speak with a member of our team or fill-in our online contact form.