Before You Rely on the DNA: Five Critical Issues for 2026

Modern forensic science is powerful, but its limitations matter more than ever.

DNA and forensic evidence can be compelling, but only when it is properly collected, analysed, interpreted and reported. As forensic science becomes increasingly sensitive and complex, the risks of overstatement, misinterpretation, and evidential overreach increase. Below are the top five forensic DNA issues we believe criminal lawyers should be alert to when considering the value of the DNA evidence in their criminal matters for this year.

1. Trace DNA: Still One of the Most Problematic Evidence Types

Trace DNA — where the biological origin is not known — is one of the most contentious forms of forensic evidence. It is generally not possible to determine how or when this DNA was deposited, and it is widely accepted that DNA can transfer indirectly and persist in the environment for extended periods. Presence alone does not equate to participation.

2. Investigator-Mediated Contamination

Modern DNA testing is extraordinarily sensitive — so sensitive that preventing contamination in the field has become increasingly challenging. Inadequate glove changes, poor scene discipline, or improper handling can readily facilitate DNA transfer between items or individuals. Investigator-mediated contamination is more common than many reports acknowledge and should always be explored.

3. Undisclosed Limitations of Forensic Tests

Forensic reports do not always clearly articulate the limitations of the tests employed. Many presumptive or screening tests are not specific. For example, tests commonly described as detecting saliva may also react to urine and other biological fluids. Understanding what a test cannot distinguish is just as important as what it claims to detect.

4. Low Likelihood Ratios = Low Evidential Value

A likelihood ratio (LR) is not automatically powerful simply because it exists. Low LRs indicate weak support for one proposition over another and should be treated with appropriate caution. Reports sometimes present such results in technical language that can mask their limited evidential weight. Always consider whether the LR meaningfully advances the prosecution case or merely adds noise.

5. Non-Probative Samples

Not all DNA is meaningful. Items such as cigarette butts, drink containers, or other discarded objects found outside a crime scene may have little or no probative value. These items often lack a clear nexus to the alleged offence and can reflect entirely unrelated, innocent activity. The key question is not whose DNA is it? but what does its presence actually prove?

Forensic evidence should assist the court, not mislead it. A careful, critical reading of forensic reports and associated data is essential to ensuring that scientific evidence is placed in its proper evidential context, so that it can be presented fairly and any limitations disclosed. If you would like assistance reviewing forensic reports and forensic data, identifying limitations, or obtaining independent expert advice, we are always happy to help.

Further Reading

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