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DNA Evidence: What Patrol Officers Need to Know

Properly collecting and processing new smaller DNA samples is often the key to solving crimes.

February 03, 2011  |  by David Spraggs

DNA Mixture

Trace DNA collection poses certain challenges to law enforcement and forensic laboratories. Because the sample size is so small, the likelihood of collecting various people's DNA in one sample increases. These mixtures make it challenging for laboratories to resolve individual contributors.

To complicate matters, trace DNA often involves collecting DNA from objects that many people may have touched. It's not uncommon to develop DNA profiles from people (known or unknown) that have nothing to do with the crime. This can open the door for defense attorneys to argue an "alternate suspect" defense. Additionally, as the sample size shrinks the possibility of contamination is increased.

Therefore more emphasis must be placed on proper collection techniques and procedures, as well as minimizing scene contamination. This starts with the patrol officer.

First, Do No Harm

Law enforcement agencies differ. Some patrol officers routinely handle crime scene investigation and evidence collection, while others call in the crime scene unit on every case. Regardless of what end of the spectrum you fall on, the one thing we can all agree on is that we don't want to contribute our DNA at the scene.

I think about this every time I enter a scene, whether it's a burglary, sex assault, or homicide. The first rule of medical ethics is, "First, do no harm." I often think of this as I process crime scenes. My goal is to leave things as is by trying to minimize scene contamination. DNA contamination is a real concern.

Every scene is different, but some basic steps will help minimize contamination. Gloves are a must. Whether leather, latex, nylon, or nitrile, gloves will prevent you from depositing DNA on items in the scene. If you're not wearing gloves, minimize your contact with items within the scene; if you don't touch an object the odds are good your DNA won't show up on that object. Small things like sneezing could also transfer DNA, so this is something to think about.

Also, patrol officers are always first on scene and they have the authority to control everything within that area. Take active steps to ensure that all other people and personnel in the scene follow these guidelines. These are just a few of the matters you'll need to think about.

Who Handles It

The next question is whether the DNA evidence is best collected by the patrol officer or whether specialized crime scene technicians are needed. Or is the item of evidence best carefully packaged and delivered to the crime lab for DNA collection? There are no strict rules. I generally prefer that the item of evidence be collected and carefully packaged, if this is reasonable. This way the state lab can conduct trace DNA collection in a controlled setting with its specific validated techniques.

However, not all items can be packaged and delivered to the lab. Patrol officers will still need the basic tools and techniques to collect DNA evidence on scene.

The good news is that most biological evidence is collected in a similar manner-whether it's blood, semen, saliva, or possible trace DNA. I'm talking about DNA evidence that needs to be collected from a surface, not a cigarette butt or beer bottle discarded by the suspect. These items are just collected and packaged and the lab handles the rest.

Using Swabs

I'm also seeing a lot of patrol officers now collecting buccal (pronounced "buckle") swabs. These are cheek swabs that are used to obtain a person's DNA profile. Buccal swabs are collected from victims, witnesses, suspects, etc., either by gaining consent or via a court order.

Buccal swabs are easy to collect. I place four swabs (some folks use just two) inside the person's open mouth. I then vigorously rub the inside of the cheek to liberate skin cells from the cheek surface to the swab. There is no need to wet the swabs prior to collection, but they should be dried before being packaged.

The basic supplies needed for DNA evidence collection are latex or nitrile gloves, medical grade sterile swabs (individually packaged or two to a package), distilled water, clean envelopes, and packaging material. I also like swab boxes. These are small boxes that the swabs are placed into after collection. The boxes have air holes and areas in which to write the date and time of collection, case number, etc.

The procedure is fairly straightforward. Once an area is identified for collection-whether a blood stain, possible semen stain, or item for touch DNA-wet a minimum of two sterile swabs with distilled water (I usually use four swabs because I feel like you can't collect too much DNA). I prefer small individual ampoules of distilled water that are discarded after one use because this minimizes the potential for contamination.

Once the swabs are moistened, proceed to rub the swabs in the stain or on the area that you want to collect DNA from. Air dry the swabs, then package them in clean envelopes or swab boxes. Then place this inner packaging in another larger envelope that is sealed with evidence tape.

It's important to minimize cross-contamination by changing gloves and equipment for each sample that is collected. Once the item is collected it should be entered into property and evidence using local guidelines and protocols regarding refrigeration.

In many criminal investigations, DNA evidence is critical to both identifying the perpetrator and ruling out people that weren't involved in the crime. The national DNA database will continue to grow as more and more convicted offender and arrestee DNA profiles are entered into the system. This will lead to a significant increase in the number of DNA hits.

I also believe that patrol officers will continue to play a significant role in DNA collection as the technology becomes a larger part of police work. 

David Spraggs is a major crimes detective and certified bomb tech for the Boulder (Colo.) Police Department. He is a member of the POLICE Advisory Board and a frequent contributor.

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