Monday, April 10, 2006

DNA Cross-contamination

This might be of interest, but then again maybe not. An issue came up recently at the office, actually it came up a while back but we have been working on the solution for us to use so it seems more recent. This issue was brought up during some of the ongoing training my deputies go through and it really makes sense when you do think about it. That issue is the possibility of DNA contamination in evidence we collect.

For years our office has been using a “drying closet” (passive air) to dry wet “evidence”, e.g. bloodied clothing from a homicide victim. Although it wouldn’t happen often there were times when there might be items “belonging to” more than one individual in the closet at a time (including the DNA sample card we obtain and keep on our “clients”). That no longer happens. We have also improved our procedure for cleaning and sanitizing the closet after each use.

As a part of the criminal investigation of some of the cases that “go through” our office, DNA testing is done on items that we collect at the scene of death based on our responsibility for collecting all evidence on and touching the decedent’s body. The DNA testing will be looking for the perpetrator’s (or someone other than the decedent’s) DNA on the person, clothing, bed linen, etc of the decedent. The DNA evidence can be easily contaminated and one of the places that can occur is while wet items are drying. If you have someone else’s DNA (blood) drying in the same closet/cabinet, it is not a stretch to imagine some that dried blood (with its DNA) floating onto the other item that is drying and “contaminating” it. Because of the way DNA is tested for with microscopic amplification techniques, it doesn’t take much of a contaminant to raise questions and potentially change the outcome of a case.

While this issue may become the plot of a CSI episode, it is a truly important part of real-life forensic science, as well as one of the “ah ha” moments in the Coroner’s “Biz”.


Anonymous said...

Can you elaborate on "not a stretch to imagine some that dried blood (with its DNA) floating onto the other item that is drying"?

Dr. Richard Keller said...

As a “patch” of blood dries, it would be easy to imagine a small flake breaking loose and floating elsewhere on the current of air.