More Sensitivity Requires More Samples

One of the greatest challenges of DNA has been to avoid contamination, specifically trying to avoid the DNA analyst or the person collecting the evidence from leaving their own DNA on a sample. There are many opportunities for this type of contamination to occur, whether it happens when the evidence is first collected or during the many techniques of DNA analysis.



Article contributed by a guest writer with six years experience as a DNA Analyst


Thankfully, these sorts of events are currently rare. Even when they do occur, labs have internal databases to help prevent a DNA analyst’s profile from being mistaken as a suspect’s. However, the same cannot always be said for criminalists and law enforcement personnel outside of DNA that still run the risk of contaminating evidence with their DNA. Many departments often have issues with obtaining references from criminalists belonging to other units or police officers and detectives. Whether it’s due to a lack of opportunity to collect a reference sample or a flat out refusal to provide their DNA, internal databases can have holes that prevent analysts from properly identifying a DNA profile as contamination.

As DNA is required to move to newer kits, we also move to increased sensitivity. The new kits are able to develop full DNA profiles from as little as 70 picrograms of DNA. Though this is a fantastic boon for low level samples, the increased sensitivity also means a higher risk of picking up contamination. If we’re able to obtain full profiles from 12 cells, it is quite possible that DNA from criminalists or detectives that previously went undetected is now perfectly visible. Since the risk of contamination is potentially increasing, we need to expand our internal databases to include all possible sources of contamination. Unfortunately, this argument hasn’t convinced many detectives or other criminalists to turning in swabs.

So, we stand at a bit of an impasse. With newer kits, touch DNA samples can become useful evidence. But without an internal database that contains all the profiles of everyone that collects or processes the sample, we can never be entirely sure that an unknown DNA profile doesn’t originate from the lab or police officers. 12 cells are an incredibly small amount considering that single drop of saliva from someone collecting evidence can contain as many as 400,000 human cells. We can have extensive training for officers and criminalist in the field to avoid contamination and wear face masks and gloves, but we can never be certain contamination isn’t occurring without checking their profiles. The increased sensitivity requires that we have references of anyone that comes into contact with crucial evidence.

Ultimately, the solution is to find out why so many officers and even a good portion of criminalists are hesitant to give references when asked. Are they worried that they’ll be attributed to contamination now that we can identify it? Or is it just a general distrust and fear of having their DNA forever stored and accessible in “some” database? Even when we argue that having their profile helps with solving casework and is only used for cross checking samples, it doesn’t always convince everyone. It seems that the only way is to require that references are given and are no longer an optional contribution. But good luck trying to convince your department that.