Contamination – should my client be concerned?

What is contamination?

Forensic experts reserve the term contamination for transfers of trace evidence which occur after the alleged crime event or incident and are not related to the crime.  Transfer before the incident is called ‘adventitious transfer’, for example someone handling a knife whilst doing some cooking before the knife is used by another person a few hours later to stab someone.

It does not just apply to DNA, any substance of forensic interest can be a contaminant, for example body fluids, fibres, hairs, gunshot residue, paint and glass fragments.

How does DNA contamination occur?

Individuals, other items or samples, equipment, reagents and consumables can potentially be  sources of DNA or body fluid contamination. For example, an investigator could inadvertently deposit their DNA at a crime scene because anti-contamination techniques and protective clothing were not used properly. An investigator may cause contamination by using equipment that had not been effectively cleaned or by using consumables including plasticware  and swabs that were not manufactured to a “DNA-free’” standard.

What can be done to prevent it?

There are recommended procedures set by the Forensic Science Regulator that police, laboratory and medical staff should follow to reduce the opportunity and likelihood of a contamination event occurring. However current forensic techniques are extremely sensitive and can detect minuscule amounts of forensic traces. Even when all of the procedures and precautions are in place, it can still occur.

Mitigations such as good laboratory design, controlled environments and working practices are crucial. The wearing of protective clothing and the use of consumables which meet a ‘DNA-free’ standard help to minimise the risk, but cannot completely prevent contamination. Different police or medical personnel should deal with the retrieval of samples from the complainant and suspect, or from different addresses.

Can it be detected?

The detection of DNA contamination is based on the routine checking of results against elimination databases containing profiles of laboratory staff, police, medical and manufacturing personnel together with profiles of recorded unsourced contaminants.  There are also systems in place to cross check results in the laboratory and detect possible contamination events.  All crime related profiles are routinely checked by the DNA laboratory against the elimination databases, however there will be some undetected instances.

If undetected, it could result in a miscarriage of justice with the wrongful exclusion or inclusion of a suspect. There have been notable contamination events reported in the press in recent years, including the MI6 spy inquest in 2010 where DNA from a crime scene examiner was inadvertently introduced into a crime sample and a case where the Forensic Laboratory incorrectly re-used a reagent tray resulting in contamination which incorrectly linked a suspect from an affray case to a rape.

Even when contamination is detected it can complicate the interpretation of the findings and occasionally mean that the true result is lost.

Is contamination relevant to my case?

In a case involving DNA evidence which implicates the defendant in a crime, our expert will scrutinise various aspects as instructed by the client.  This may include review of the reliability of the DNA match, consideration of the likelihood of an ‘innocent’ transfer of  DNA (either direct or indirect). Consideration will also be given to the possibility of contamination having occurred during the sample’s retrieval, examination and testing, this might include:

  • Was the sample collected using appropriate anti-contamination techniques?
  • Was it packaged and sealed correctly, was the continuity maintained?
  • Was the sample examined and tested using appropriate procedures to mitigate against contamination?

For example, if a police officer seized an item related to a crime, were they wearing protective clothing?  If they were searching a scene,  did they change their gloves prior to touching  the relevant item?  Was the item packaged and sealed immediately? If the correct procedures were not followed, it is possible that an inadvertent transfer of DNA from another item or surface, i.e. contamination, might have occurred?

If you are concerned that contamination may be an issue in your case, contact our experts for advice.