How reliable are mixed DNA results?

Mixed DNA profile results can be informative, but they are generally more complex — and sometimes less certain — than results obtained from a single-source DNA sample.

A mixed profile contains genetic material from more than one person. The DNA from different contributors may overlap, vary in quantity, or be present at low levels, making interpretation more challenging and dependent on analytical decisions.

Modern laboratories use statistical methods and probabilistic genotyping software to evaluate mixtures. In Australia and New Zealand, STRmix is commonly used for this purpose, while other jurisdictions may apply different systems. These tools model possible contributor combinations and calculate the strength of support for a particular individual being included in the mixture.

Although scientifically sophisticated, the reliability of mixed DNA results depends on several factors, including the quality of the DNA, the number of contributors, the amount of genetic information available, and the assumptions used during modelling. Different assumptions or analytical approaches can lead to different evaluative outcomes.

Errors and uncertainties may also arise where mixtures are complex, where DNA is present in small quantities, or where contributors cannot be clearly separated. The results may therefore be expressed in probabilistic terms rather than as definitive identifications.