Skip main navigation

Contamination in DNA profiling

In this video, Dr Vivek Sahajpal and Dr Gavin Turbett deal with the issue of contamination and its impact on forensic DNA profiling.
One of the common problems which is encountered during DNA profiling is the problem of contamination. The forensic DNA analysis technology is a very, very sensitive technique which is highly sensitive and capable of amplifying and detecting low quantities of DNA. This increases the chance of detecting contaminant DNA. The contamination can happen during any step, starting from the point of collection to the analysis. Contamination may lead to erroneous results. There can be several sources of contamination. Some of them are listed here. The sample collectors can themselves contaminate the sample. Similarly, the laboratory staff can also be a source of contamination, and the reagents and consumers can also be a source of contamination. And similarly, they can also be sample cross contamination.
Contamination at the scene of crime. This can occur at several points, including at the crime scene, during collection and transport of evidence on submission of evidence, during analysis, on final stage of final storage of the exhibits and accordingly need to take steps to prevent this contamination. Contamination can occur due to natural weather conditions, poor seam control, number of personnel attending the crime scene, lack of knowledge of priority, poor collection techniques. Example dusting prior to DNA collection, leakage of wet exhibits, etc., may lead to contamination of the evidence at the crime scene and after that. So in order to prevent contamination, lab should assess its specific needs both technically and administratively prior to establishing a process.
To demonstrate that facility is designed to minimise contamination. Restricting the movement of staff equipment and consumables between pre-amplification and post -amplification areas. Training of staff. Quality control. Testing of reagents and consumables. Storage and treatment of consumables. And implementation of clean techniques. So in the laboratory itself, we prefer to use the term contamination minimization, not contamination prevention. If you talk about contamination prevention, it gives the impression that it can absolutely be prevented. And in reality, I do not believe that it can. I think it’s safer to talk about minimization because that acknowledges the possibility of contamination. It acknowledges the possibility that it may actually occur, whereas prevention suggests that it can be stopped completely.
So in terms of what we do to minimize the risk of contamination, it’s the work practices that we employ and having well trained staff again wearing PPE, gowns, gloves, face masks and hair nets at all times. Gloves are changed between every time the exhibit is handled. We may go through dozens of pairs of gloves just looking at one piece of clothing. Again, avoid talking, coughing or sneezing. If a staff member actually has a cold, we actually roster them into a different part of the laboratory away from the lab areas so that they’re decreasing the risk of contamination. We try to handle the actual exhibit as little as possible. Use tweezers, use forceps to actually move the exhibit around.
The laboratory areas themselves are highly restricted. Most staff do not ever go into the lab areas unless they are specifically working in there. Certainly, a lot of staff would never go into the lab areas. So clerical staff, a lot of the support staff wouldn’t go into lab areas at all, ever. Where possible, we also try to purchase and use consumables such as the plastic tubes and things that are actually being treated and are certified by the manufacturer as being DNA free. We try to keep the lab clean and uncluttered. The work surface is cleaned after every exhibit, and we use new tools between every exhibit as well.
This is even if we’re looking at multiple items of clothing from the same person, we will still clean down the work surface between every single item. Every, probably about every three months, the laboratory area is shut down for a few hours and we go through an aggressive deep cleaning process where everything is swabbed and wiped down and with bleach and cleaned, and then we start working again. The idea is to get rid of any background DNA that might be starting to creep into the room. We have completely separate work spaces for and equipment for both crime scene and reference samples. We don’t ever process reference samples in the same location that crime scene samples are processed.
That is an obvious risk and source of contamination. We maintain an elimination database for all forensic workers and support staff. So this includes the actual forensic laboratory scientists, but it also includes our cleaners because they do go into the lab areas and our IT staff. We have regular visitors, in particular the technicians who would come into service the laboratory equipment. They provide DNA samples. The forensic police, people like forensic pathologists and mortuary staff should also ideally be providing reference DNA samples for inclusion on an elimination database. Ideally as well, supplies of consumables should keep a database of their staff. Now, they may, of course, not be willing to share that database with the laboratory.
But they should keep the database for their own records. And if we have a contamination event, we find a profile that we can’t explain, one line of investigation that we will pursue is to go back to the manufacturer and ask them if that DNA profile that we’ve seen actually matches one of their staff. If it does, of course, then we’ve established the source of that DNA and can recognize it for what it is.

In this video, Dr Sahajpal and Dr Turbett deal with the issue of contamination and its impact on forensic DNA profiling. It is essential for legal professionals to understand the sources of contamination at different stages of DNA profiling, its impact and how it can be minimised. This will be useful in examining the quality of DNA evidence in case work.

This article is from the free online

Decoding Forensics for Legal Professionals

Created by
FutureLearn - Learning For Life

Our purpose is to transform access to education.

We offer a diverse selection of courses from leading universities and cultural institutions from around the world. These are delivered one step at a time, and are accessible on mobile, tablet and desktop, so you can fit learning around your life.

We believe learning should be an enjoyable, social experience, so our courses offer the opportunity to discuss what you’re learning with others as you go, helping you make fresh discoveries and form new ideas.
You can unlock new opportunities with unlimited access to hundreds of online short courses for a year by subscribing to our Unlimited package. Build your knowledge with top universities and organisations.

Learn more about how FutureLearn is transforming access to education