Forensic DNA analysis has undergone a remarkable transformation over the past three decades. Where once we required a visible biological stain to obtain a useful DNA profile, modern technology can now detect and interpret extraordinarily small quantities of DNA, sometimes originating from just a few cells.
This extraordinary sensitivity has revolutionised criminal investigations. But it has also created a systemic problem that forensic science is still grappling with.
Has DNA analysis become so hypersensitive that our collection procedures are now generating the very ‘evidence’ we seek to interpret?
When sensitivity becomes a complication
DNA is everywhere. We deposit it through ordinary contact, and it can be transferred between people, objects, and surfaces, sometimes through multiple intermediaries.
Modern DNA profiling is sufficiently sensitive to detect material that arrived through entirely innocent activities. Importantly, the presence of someone’s DNA on an exhibit does not establish that they touched it, or that they participated in the offence under investigation.
But there is another complication: the very process of collecting DNA evidence can introduce or redistribute DNA.
Consider a crime scene officer examining several items of evidence. Gloves protect the officer from biological material, but they do not prevent DNA from being transferred between surfaces. A glove that contacts one item may collect DNA and subsequently deposit it elsewhere.
The resulting DNA profile may be entirely accurate. The problem is not the identification of the DNA contributor, but the assumption that the DNA was present at that specific location before the investigation commenced.
The lesson from Piccioni
This issue came into sharp focus in State of Western Australia v Piccioni [2025] WADC 69. The case involved DNA recovered from a firearm, with the prosecution relying on a mixed DNA profile that was 100 billion times more likely if the accused was a contributor.
However, an independent review of the audiovisual footage revealed extensive forensic handling failures, including glove reuse and excessive movement of exhibits. The Court refused to admit the DNA evidence, ruling that the risk of investigator-mediated contamination was real and directly undermined its reliability.
The significance of Piccioni extends beyond police searches. It illustrates a fundamental boundary in forensic science: a highly discriminating DNA result does not establish how or when the DNA came to be on the exhibit.
Could the medical examination itself move DNA?
The potential for investigator-mediated transfer is not confined to police officers or crime scene examiners. It extends to clinical environments, including sexual assault medical examinations.
These examinations are conducted in highly controlled environments by trained medical professionals using strict protocols. Nevertheless, the physical mechanics of the examination itself can inadvertently relocate DNA.
- Internal transfer via swab: A swab passed into the vagina may collect DNA from the external genital area or lower vaginal canal and mechanically transfer it further internally.
- Redistribution via speculum: A speculum may collect external DNA on its surface during insertion and redistribute that material further into the vaginal vault when positioned or opened.
These mechanisms raise a critical interpretative question for the courts: If a complainant’s internal vaginal swab tests positive for a defendant’s DNA, can we automatically conclude that it was deposited there through penetration?
The scientific answer is no.
This does not mean DNA transfer occurred during the examination, or that the finding lacks evidential value. It means the location of the DNA must be interpreted in the total context of the collection procedure, the timeline, and the available scientific research on DNA transfer.
What does this mean for criminal lawyers?
Forensic DNA evidence is frequently accompanied by impressive likelihood ratios in the billions. But those figures only answer one question: Whose DNA is it? They do not establish how it got there.
As DNA testing sensitivity continues to outpace frontline handling procedures, legal practitioners must look beyond the lab report and rigorously examine the chain of custody:
- Who handled the exhibit, and in what precise sequence?
- Were gloves systematically changed between handling different items or areas?
- What specific instruments or speculums came into contact with the sampling area?
- Could the medical or forensic collection process itself have introduced, masked, or relocated the biological material?
These questions do not undermine the extraordinary achievements of modern forensic science. Instead, they recognize the boundaries of what technology can tell us.
Whilst science can seamlessly isolate a DNA profile from just a few cells; the true legal battleground now lies in reconstructing the human activities that moved them.
