Biological Fluid Testing in Sexual Assault Matters
Biological fluid testing in sexual assault matters is often not as black and white as lawyers would like.
In the last of this series on testing for biological fluids, we are covering the detection of semen.
Understanding Presumptive Tests for Semen
Have you ever seen the following in a forensic statement or report?
“Possible semen was detected”
“A presumptive test for semen was positive”
This means only a presumptive or screening test has been performed, which generated a positive reaction. More often than not, there is no further information about these tests included in the report.
These statements do not mean that semen has been detected. Furthermore, even the tests used to confirm the presence of semen also react to other biological fluids as well as other commonly found substances.
How Does Semen Testing Work?
Testing for the presence of semen relies on the detection of an enzyme known as acid phosphatase. Remember that an enzyme is a molecule that has a biological function.
This enzyme is present in high levels in semen, but it is also found in other biological fluids, such as:
- Urine
- Faeces
- Vaginal secretions
Additionally, it can be found in some plant extracts (such as cauliflower) and yeast.
For the Scientifically Minded: The Reaction Process
One of the reactions used in testing works as follows:
sodium alpha-napthyl phosphate ------> sodium phosphate + napthol
(enzyme)
sodium phosphate + napthol + buffered brentamine ------> purple AZO dye
This is known as an enzymatic reaction, as it relies on an enzyme present in the material to cause a reaction. The reaction generally results in a colour change. In relation to semen testing, this enzyme is acid phosphatase.
What Does a Positive Result Actually Mean?
The good news is that the primary confirmatory test for the presence of semen is highly conclusive—this is the observation of spermatozoa using a microscope.
In addition, sometimes it is possible to provide further opinions based on the state and number of spermatozoa observed. This is known as a time since intercourse evaluation. Stay tuned for more on this in the next newsletter.
Misinterpretation of Presumptive Tests
As lawyers, it is not appropriate to assume that semen has been detected based on a positive reaction to a presumptive test, especially when it comes to samples collected as part of a SAIK or MEK.
It is imperative to find out the limitations of this testing to prevent misleading evidence from being presented in your matter.
Presenting the Limitations of This Evidence in Court
The limitations of these tests are frequently absent from forensic reports and statements. Their absence can allow undue weight to be applied to the testing.
It is imperative to reveal these limitations to the court to prevent an over-evaluation of the evidence. IFS stands ready to help pinpoint these limitations, ensuring your case evidence receives fair and impartial evaluation.
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Specialising in DNA analysis, STRmix™, biological fluid examination, and bloodstain pattern interpretation, Jae now works independently with criminal defence teams to review and challenge forensic evidence. Her mission is simple: to ensure the science presented in court is accurate, balanced, and truly understood.