Feb 01 2016

Database Searching – Taking Mixtures Further than Ever Before

DNA MixturesForensic

At The Institute of Environmental Science and Research (ESR) we receive approximately 2600 volume crime samples per year and conservatively it is estimated that approximately 45% of these are mixed. These mixtures range from profiles where it is relatively easy to resolve a DNA profile suitable to load onto the Crime Sample Database by conventional means, to complex mixtures that previously offered little in the way of progressing a case without a suspect reference sample. As my main role at ESR is carrying out casework specifically on volume, suspectless cases, I was excited by the possibilities that the Database Search capability STRmixTM opened up for the complex unresolvable mixtures and I have been involved in several pilot projects with Police districts within New Zealand.



Written by: Laura Russell, The Institute of Environmental Science and Research (ESR)



At ISHI 26 in Grapevine, Texas, I presented a poster on a review of cases that had used the Database Search capability of STRmixTM. This capability allows information from unresolvable mixtures to be compared to a reference database. Thereby, potentially enhancing the provision of intelligence information to Police. My colleague, Kate Stevenson (Senior Forensic Scientist, Institute of Environmental Science and Research – ESR, New Zealand), and I reviewed all the cases that had been entered onto the Crime Mixtures Database over the first eighteen months since it was implemented. This was done with the aim of determining an approximate success rate of the capability and to determine whether there were any broad profile classes that the database entries could be segregated into.

One hundred and ten cases were found to have been entered onto the Crime Mixtures Database since the introduction of the technique. From 71 mixtures entered from serious crime cases 56 were found to have linked to individuals on the DNA Profile Databank. From 39 mixtures entered from volume crime cases (property crime) 36 of these were found to link to an individual.

Three different types of profiles were found to have the technique applied to them:

  • Mixtures where two or more individuals had contributed DNA in approximately equal amounts.
  • Mixtures where it was possible to resolve information at some loci, but the information did not meet the conventional load criteria of the laboratory.
  • Mixtures where it was possible to assume the presence of a contributor (i.e. the complainant or the DNA profile obtained from an elimination reference sample.)

Database Searching, enabled by the use of  STRmixTM, is a powerful tool in the provision of intelligence information. It is sustainable and through careful consideration of the data presented to them, Scientists at ESR have been able to progress cases further than was previously possible without a suspect reference sample.


Database Searching can enhance the provision of intelligence information to Police from mixtures. Click To Tweet


As a team, we look forward to promoting this capability to members of the New Zealand Police and increasing the number of mixtures we are able to provide intelligence information on.


The Institute of Environmental Science and Research (ESR) employs around 400 expert minds whose independent scientific advice and services help improve our collective economic and social wellbeing. Using the largest team of forensic, social, radiation, environmental and infectious disease scientists and epidemiologists in New Zealand they deliver solutions for local and central government, industry and other science organisations. Their expertise, experience and networks are trusted to solve complex problems, collect, collate and analyse huge amounts of data and manage a range of critical national science assets and facilities for New Zealand.