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Updated: Jan 20, 2021

Fernanda Henry[a][b]


DNA analysis of biological evidence has the potential of eliminating possible sources of a sample, or in the case of matches, indicating what percentage of the population has the same characteristics. The ability to successfully perform DNA analysis will often depend on how well the evidence is collected and how well they are preserved.

Biological evidence refers to samples of biological material—such as hair, tissue, bones, teeth, blood, semen, or other bodily fluids—or to evidence items containing biological material. This biological evidence should be retained in an appropriate storage facility until needed for court or for forensic testing. Such evidence is frequently essential in linking someone to or excluding someone from crime scene evidence. Biological evidence should be properly preserved, processed, stored, and tracked to avoid contamination, premature destruction, or degradation. An apparent, unbroken chain of custody must accompany all exhibits.

General Evidence Handling:

1. Keep all body fluid evidence as cool as possible to minimize degradation.

2. Package all body fluid evidence separately and in paper. DNA analysis is very sensitive and will detect contamination of evidence packaged together.

3. Never use plastic bags or containers to store stained articles. Be sure body fluid stains are air dried before packaging.

NB If drying wet evidence is not possible, place the evidence in an impermeable, nonporous container and place the container in a refrigerator that maintains a temperature of 2 °C – 8 °C (approximately 35 °F to 46 °F) and that is located away from direct sunlight until the evidence can be air dried

4. Bacterial action is accelerated under damp conditions. Bacteria can destroy DNA and make analysis challenging, if not altogether impossible.

5. Seal each package with evidence tape or other seals, such as heat seals; do not use staples.

6. Mark across the seal with the sealer’s identification or initials and the date. At a minimum, the following should be on each package:

i. a unique identifier,

ii. the identification of the person who collected it

iii. the person from whom/or place where item was collected

iv. the date of collection

Safety Precautions:

Individuals handling any evidence should assume that all of it may contain potentially hazardous biological material. Anyone handling biological material may be exposed to harmful infectious diseases. The following section discusses procedural implications related to the safe handling of biological evidence and guidance on the way individuals should protect themselves.

Universal Precautions:

The U.S. Occupational Safety and Health Administration (OSHA) developed universal precautions to protect workers from exposure to human blood or other potentially infectious materials. It is not possible to determine if every bodily fluid or stain collected from crime scenes is contaminated with a blood borne pathogen; therefore, all bodily fluids and tissues are presumed to be contaminated. When individuals handle any type of biological evidence, procedures need to be in place to reduce or eliminate the risk of exposure to blood borne pathogens that can transmit disease (OSHA 2012).

Common diseases/viruses caused by exposure to blood-borne pathogens include Hepatitis and Human Immunodeficiency Virus (HIV). These raise the most concern because of the potential for lifelong infection and the risk of death associated with infection once an individual is exposed.


The appropriate use of PPE is intended to protect the individual and the evidence from cross-contamination. PPE includes disposable gloves, disposable overalls, laboratory coats, masks, and eye protection.

PPE should be used in every situation in which there is a possibility of exposure to blood or infectious diseases. Gloves and protective clothing should be worn when providing first aid or medical care, handling soiled materials or equipment, and cleaning up spills of hazardous materials. Face protectors, such as splash goggles, should be worn to protect against items that may splash, splatter, or spray.

PPE must be clean and in good repair. PPE that is torn or punctured, or that has lost its ability to function as an effective barrier, should not be used. Disposable PPE should not be reused under any circumstances. While using PPE, individuals should not touch their eyes or nose with gloves.

PPE must be removed when it becomes contaminated and before leaving the work area. Used protective clothing and equipment must be placed in designated areas for storage, decontamination, and disposal.

Dried blood or other dry potentially infectious material should not be assumed to be safe. PPE should be used when handling these items. Use bio-hazard labels and signage on packages with biological evidence.


There are two physical states in which biological evidence can be submitted: wet and dry.

Wet Storage

All evidence that is wet should be dried to be properly stored and tested in the future. Drying wet items of evidence, such as blood-soaked garments, should be the first task when handling wet biological evidence once it has been collected.

Dry Storage

The presence of water encourages the growth of yeast, mold, and bacteria, which can degrade DNA. It is therefore important to store DNA evidence in a dry, water free environment.

Paper Evidence Bags

Homicides, sexual assaults, aggravated assaults, robberies, and burglaries frequently involve bulky evidence, such as clothing and bedding, these must be stored in paper bags.

Evidence Boxes

Evidence boxes can also be used for large and oddly shaped items.

If firearms are to be analyzed for biological evidence, the submitting individual must ensure that the box is sealed and must indicate on the exterior of the box that the weapon was unloaded and made safe and may contain biological material. Alternatively the firearm can be swabbed using 100% cotton swabs and the swabs submitted to the laboratory.

Evidence Envelopes

Small items of evidence (e.g., trace evidence, cigarette butts, fibers, etc.) may be stored in small envelopes. Different sizes of envelopes can be selected based on the dimensions of the items.


Sexual Assault Kits

The sexual assault kit package is an envelope. An itemized list of collected items should be submitted with the kit. Sexual assault kits are often retained for decades and must be stored in a manner that prevents degradation and facilitates easy retrieval and identification. The kits can be stored at room temperature or in a temperature- and humidity-controlled facility.

Tissue Samples

Tissue samples that are submitted for DNA analysis are usually stored at -20 °C as rapidly as possible to halt the degradation process.


[a] Director, Saint Lucia Forensic Science Laboratory [b] 1st Vice President, Caribbean Association of Forensic Sciences (CAFS)

[1] S. M. Ballou, M. C. Kline, M. D. Stolorow, M. K. Taylor, S. R. Williams. The Biological Evidence Preservation Handbook: Best Practices for Evidence Handlers. National Institute of Standards and Technology (NIST). 2013

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