Transfer and Contamination in Sexual Assault Matters

The complexities of DNA in sexual assaults.

Transfer, contamination, and missing analyses can drastically alter the weight of DNA evidence.

Following my presentation to LegalWise Seminars last week, I thought it was timely to reinforce this presentation and re-visit some of the complexities of forensic biology evidence in sexual assaults.

Forensic DNA and biological evidence is frequently regarded as the most persuasive form of forensic evidence in sexual assault matters. Jurors often perceive DNA results as conclusive, creating a significant evidentiary advantage for the prosecution. However, the scientific realities underlying DNA findings are considerably more complex. In particular, the detection of seminal fluid, observation spermatozoa, the risks of transfer in the sample process and contamination events present serious limitations that criminal defence lawyers must appreciate when preparing their cases.

Contamination and transfer concerns

Although compelling evidence, the presence of a small number of spermatozoa or DNA in the vaginal cavity may not be evidence of penetration. A scientific study conducted in 2013 demonstrated that the nature of collecting intimate samples in a sexual assault matter can push external material into the vaginal cavity, creating the appearance of penetration.

This occurs 100% of the time if a speculum is used and up to 86% of the time when the samples were collected blind [Loeve et al 2013].

However, this study has limitations that the samples were not collected in the same manner as samples in sexual assault matters. It demonstrates that transfer of material occurs.

A 2025 study using real world sampling methods on anatomical models, has demonstrated that >25% of the time, practitioners transferred DNA from the external vaginal area onto the high vaginal swab. This study also showed that there was a transfer of material to other sampling sites — this occurred >50% of the time — and more than 10% of samples contained unidentified DNA [Gaskell et al 2025].

Detection of seminal fluid

Often samples contained in sexual assault investigation kits are not tested for the presence of seminal fluid — they jump straight to microscopy. However, it is important to note that semen consists of seminal fluid + a cellular component (the spermatozoa). Testing for the presence of seminal fluid has its own limitations. This test can produce false positives with other biological fluids, such as vaginal secretions and faecal material. However, when used in appropriate case circumstances it can assist in the interpretation of the biological evidence.

Observation of spermatozoa

Forensic reports usually record only the presence of spermatozoa. They do not disclose number or state of spermatozoa observed. This information can be essential to evidence evaluation as the presence of a small number of spermatozoa, and spermatozoa without tails may not be evidence of a recent ejaculation. Scientific data indicates that finding spermatozoa without tails 24 hours after deposition is very low [Casey et al 2017].

The reporting of spermatozoa without their number or state can present misleading evidence. A handful of sperm cells, without tails, may be consistent with historic or indirect transfer, not recent penetration or consistency with the allegation. Therefore, it is critical to know the number and state of the spermatozoa detected, the time from alleged incident to sample collection as well as any previous consensual sexual encounters.

Transfer of spermatozoa

Sperm and seminal fluid can transfer via laundry, bedding, or indirect social or household contact. Alternative explanations for the evidence — or limitations to the evidence, such as the transfer of spermatozoa in the laundry — are not disclosed in the State Service Provider reports. However, these limitations should be disclosed to ensure a complete picture of the evidence can be evaluated by the court.

Biological evidence in sexual assault matters is complex and the issues are significantly more complex than a forensic DNA report indicates. I have been reporting and reviewing sexual assault evidence for over 20 years. Before you advise your client to plead guilty or allow the evidence to be presented at trial, please contact me to ensure you know exactly what you are dealing with.

Further Reading

• NIST — Evaluation of Forensic DNA Mixture Evidence: Guidelines for Evaluation, Interpretation and Reporting
• NIST — DNA Mixtures: A Forensic Science Explainer
• SWGDAM — Interpretation/Validation Guidelines (PDF)