Collecting DNA Evidence

Crime Scene Forensics,LLC

STR DNA technology changed the rules of evidence handling:
1. STR DNA increased sensitivity in testing
2. STR DNA increased awareness in evidence handling
- As a result, the crime scene investigator must take great precautions to avoid
the contamination of DNA evidence

In the past, all crime scenes needed to be protected against elements such as: Weather, Animals,
and Traffic. With the inception of STR DNA testing, the crime scene investigator must also be
concerned with scene contamination by crime scene personnel.

It is imperative that crime scene personnel take precautions to avoid contamination of DNA evidence:

1. Personal Protective Equipment (PPE) should be worn at all times.
- a single hair or drop of sweat from an unprotected person could leave an unknown DNA
sample at the scene
2. When collecting DNA samples, the crime scene investigator should change gloves and forceps
after each item is collected, in order to avoid cross contamination between items of evidence

Documenting and Collecting DNA Samples:

1. Photograph stain/sample with and without a scale
2. Carefully swab the stain
- preferably with no dilution
- if stain is dry, use one drop of distilled water on the swab
3. Allow to air dry before packaging
4. Carefully label & seal (date, time, initials).
5. Maintain the chain of custody.
6. Provide information that you think may be important.
- Note if items were collected from an unusual location or might have
possible contaminants.
7. Forward to the forensic lab ASAP

The proper collection and storage of biological evidence for DNA
testing includes the following:

1. Biological evidence should be allowed to air dry before
packaging.
- ideally, it should be hung up in a clean dry room,
away from direct sunlight
2. Biological evidence should be packaged in paper bags.
- paper breaths, and allows the item of evidence to
remain dry.
- Plastic bags do not breath, and can cause moisture
and mold to grow on the evidence, which in turn can
have a negative affect on DNA testing
3. Biological evidence should be stored under laboratory
conditions as available resources permit - or in a
cool, dry climate, free of moisture
4. Place liquid items in collection tubes and refrigerate

Before a DNA test can be performed, a control sample should be
obtained from the victim, suspect, and any other persons whose DNA
may be found on the item of evidence.

A control sample can be in the form of whole blood, a buccal swab, or
any other known exemplar from the person in question. The easiest
type of DNA control sample to obtain is a buccal swab. A buccal swab
involves swabbing the inside of a person's cheek with an approved
type of swab, for about 30 seconds, to secure the buccal cells that are
found on the inside of the cheek. If done properly, a full DNA profile
can be obtained from this swab.

–Before obtaining DNA control samples, consider having the person
in question sign a DNA consent form.

The fascinating world
of forensic
science...made simple