Investigations | Section 3.3

Fundamentals of Criminal Investigation by Adam J. McKee

Section 3.3: Blood and Biological Evidence

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, which may or may not have been previously analyzed at a forensic laboratory, 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. The criminal justice system depends on presenting evidence to judges and jurors to help them reach a conclusion about the guilt or innocence of the defendant.

All criminal justice stakeholders, including law enforcement officers, lawyers, forensic analysts, and fact finders, should be certain that the biological evidence they are considering has been properly preserved, processed, stored, and tracked to avoid contamination, premature destruction, or degradation. In addition, individuals who come into contact with biological evidence, such as evidence custodians, need to be confident that it has been packaged and labeled in a way that will allow them to efficiently locate relevant evidence for a case. To establish this confidence, all handlers of biological evidence should follow well-defined procedures for its optimal preservation.


Contact with bodily fluids can spread disease such as those caused by bloodborne pathogens, and individuals handling biological evidence should treat it as hazardous to ensure safety. This section offers recommendations on various aspects of biological evidence handling, including the use of personal protective equipment.  

Identifying Biological Evidence

Existing state laws vary in their definitions of what constitutes biological evidence in the context of evidence retention. A review of the National Institute of Justice’s list of items from which biological evidence can be found for criminal cases illustrates the variety of items that can be successfully tested with current technology. Further, touch DNA, or DNA contained in shed skin cells that transfer to surfaces that humans touch, can be sampled from countless objects and surfaces.

Requiring the retention of all physical evidence that can potentially contain DNA would result in the retention of all evidence collected unless it was screened to determine the possible presence of genetic material. Law enforcement organizations must attempt to balance the interests of justice with practicable storage concerns and to offer a minimum threshold for biological evidence retention.

Bulky Items

To facilitate forensic testing for trial and post-conviction proceedings, it is essential to store and track as much of the evidence as necessary. However, it may be extremely difficult to maintain large or bulky items of evidence from which biological material is derived. For the long term, agencies might find it sufficient to retain samples taken from a large item as opposed to the large item on which biological evidence may have been located Other examples of bulky evidence include a car, the wall/ceiling of a house, carpet, or another large piece of furniture such as a bed. If the origin of a sample is well documented (such as through photographs or case files), it may not be necessary to store the entire couch for testing and future re-testing.

Case Status

When determining the duration of time that biological evidence must be held, it is essential to understand what is meant by “case status” for criminal cases. Generally, there are four categories of case status:

  • Open Cases (i.e., no suspect, but investigation continuing)
  • Charges Filed (i.e., suspects charged and court proceedings active)
  • Adjudicated (i.e., conviction, dismissal, or acquittal)
  • Unfounded/Refused/Denied/No Further Investigation

Open cases.   Open cases are those in which one or more suspects have not yet been identified or charged, a suspect has been identified but not yet charged, or the investigation is ongoing. As a standard practice, it is recommended that the evidence be maintained by the holding agency for as long as the statute of limitations for the crime or as applicable by law.  Biological evidence that is collected in the course of an open investigation should be retained indefinitely for homicides and, at a minimum, for the length of the statute of limitations for all other offenses.

Charges filed.  Standard practice dictates that all evidence in any case being prosecuted is maintained in the event that the evidence is needed for laboratory analysis or court proceedings. When charges are filed, a person has been charged and court proceedings have been or will be initiated. Evidence custodians should be notified if charges have been filed to (1) communicate case status for evidence release requests and (2) assist evidence custodians in determining disposition status.  A communications link should be established between investigators, prosecutors, and the responsible custodial agency to be able to determine if charges are filed.

Adjudicated.  A case is adjudicated when a final judgment has been rendered in a legal proceeding. The disposition of evidence in adjudicated cases varies according to the crime category. Knowledge of the retention statutes in one’s state is essential.  Biological evidence should be preserved through, at a minimum, the period of incarceration in the following crime categories, as defined in NIBRS, regardless of whether or not a plea was obtained: homicides, sexual assault offenses, assaults, kidnapping/abductions, and robberies. For all other Group A and B offenses, biological evidence may be disposed of upon receipt of authorizations.

Unfounded.  In cases categorized as unfounded, refused, or denied, or for which no further investigation will be conducted, evidence can be disposed of upon receipt of disposition approval from the assigned investigator unless such disposal is prohibited by law. This category includes instances in which the victim chooses not to press charges, the prosecutor decides not to file charges, the investigator determines no arrest will be made, or the case is exceptionally cleared.  After it is determined that charges will not be sought or filed, evidence, including any biological evidence, need not be retained unless destruction is prohibited by statute.

Packaging and Storage of Biological Evidence

The packaging and storage of evidence is of paramount importance in forensic investigation. However, requests to produce evidence have demonstrated inadequacies in the packaging and storage of some evidence. Further, studies call for greater care when packaging and storing evidence to prevent contamination and to ensure reliable analysis in the future.

Multiple underlying factors affect law enforcement’s ability to appropriately store evidence for optimum preservation, including limitations in the management and capacity of the storage facility, insufficient materials available for packaging, inadequate or improper temporary storage, changes in technology, and the lag between evidence collection and transport of the evidence to the evidence storage facility.

Jurisdictions should place greater emphasis on the needs of their property rooms and staff members. The jurisdiction must ensure that the agency has sufficient resources and must apply appropriate methods and procedures to ensure that evidence is maintained in a condition suitable for future analysis.   In tandem with state or local legislatures, managers in law enforcement and relevant stakeholders should advocate for additional resources and funding to ensure the integrity of biological evidence through prioritizing the packaging, storage, maintenance, and security of the evidence in their jurisdictions.

Wet versus Dry Evidence

There are two physical states in which biological evidence is submitted: wet and dry. Certain types of evidence, such as blood-draw samples or some of the contents of a sexual assault kit, must remain in liquid form. In most cases, these types of evidence are obtained from the crime laboratory or medical facility. All other evidence that is wet should be dried to be properly stored and tested in the future. Drying wet items of evidence, such as a blood-soaked garment, should be the first task of anyone handling wet biological evidence once it has been collected.

Temporary Storage of Wet Items

At times, the evidence handler may have to temporarily store evidence in its wet state because the facilities or equipment necessary to dry it properly are not available. In such a case, the handler should place the evidence in an impermeable and nonporous container (i.e., packaging through which liquids or vapors cannot pass), such as a metal can or glass jar, and should place the container in a refrigerator that maintains a temperature of 2°C to 8°C (approximately 35°F to 46°F) and that is away from direct sunlight. The handler may leave the evidence there until it can be air dried or submitted to the laboratory.

Plastic bags can be used temporarily to store wet evidence but must not be used for long-term storage because of the possibility of bacterial growth or mold. Exceptions include plastic bags that contain desiccant, a drying agent that prevents condensation and the subsequent growth of fungi or bacteria, and breathable plastic bags (Tyvek) that can be used for damp items and swabs.

Drying Wet Evidence

If evidence with wet biological material is not correctly air-dried, there is a high probability that the biological material will be destroyed by bacterial growth. This could potentially preclude generation of DNA results. Here are a few examples of low-tech and high-tech methods for properly drying evidence.

Agencies that do not have sufficient funds or a need (i.e., they do not handle a significant volume of wet evidence) for equipment specifically designed for drying evidence generally use low-tech methods. In these cases, it is recommended that an isolated and secure area—such as a locker, shower stall, or room—be designated for this purpose. For example, a metal locker specifically labeled for biohazards is commonly used to dry evidence.

These materials will be used for repackaging the evidence once it has dried. Wet garments should hang with sterilized paper beneath and between them to minimize contamination while drying. After the drying process, the paper should be packaged separately and submitted with the garment, as it may contain trace evidence.

A shower stall is also an excellent, inexpensive way for departments with limited resources to dry evidence. Departments can create this system with a prefabricated fiberglass shower enclosure elevated on a wooden frame to make room for controlled drainage. If possible, there should be an adjacent water faucet on which to attach a cleaning hose for washing the enclosure during decontamination.

Any room dedicated to drying evidence should have surfaces that allow for easy decontamination. For example, figure III-3 shows a fully tiled room outfitted with stainless steel hanging rods. The locking mechanism on the door handle prohibits access to all except the assigned personnel.



Decontamination of surfaces or items can be accomplished by using a freshly made solution of 10 percent bleach or a suitable substitute. Individuals responsible for decontamination should consult with the laboratory for suitable substitutes (Centers for Disease Control and Prevention, 2012). Refer to the discussion on chemical treatment in section II for more information.

Liquid Evidence and Tissue

Certain types of evidence will remain in liquid form or contain fluids. These types require different types of packaging materials as well. Specific storage conditions regarding these and other types of evidence will be discussed later in the section.

Blood samples.   Generally, blood draw tubes and vials are collected and submitted in some type of container recommended by the crime laboratory and/or hospital. If the department receives a vial or tube that is not packaged in a readily identifiable manner, it should be placed in an envelope that is easily recognizable, clearly marked as to its contents, and bearing a visible biohazard label. Glass vials of blood should never be frozen because the vial might explode or crack.

Hypodermic Needles. Department packaging protocols should require that any type of needle or other sharp object entering the property room be stored in a container that is closeable, puncture-resistant, leak-proof on the sides and bottom, labeled or color-coded, and breathable. These items should not be commingled in a package with other evidence. Sharps containers also must be maintained upright throughout use.

For employee safety, syringes should be stored in an area designated for such evidence. Commingling packaged syringes with other evidence creates a special safety hazard because syringes can accidentally deliver infectious agents directly into the bloodstream (HERC 2012). Filing drawers, bins, or boxes can be used for storing these items.

Breathable Storage Containers

Throughout this section, breathable storage containers are mentioned as a preferred method for packaging. Breathable containers are important because they prevent condensation, which can encourage the growth of bacteria that can attack and degrade DNA samples. Oxygen can provide a protective barrier against these types of bacteria.

Urine Samples.  If an agency receives a vial or tube that is not clearly labeled as containing urine, it should be labeled or packaged in an identifiable envelope or box that is clearly marked as to its contents. Employee safety mandates that this type of biohazard, similar to blood, tissues samples, and extracted DNA, be segregated in one centralized location for easy identification and safe storage. Urine should not be frozen in glass jars or vials.

Sexual Assault Kits.   State or local crime laboratories, local hospitals, or evidence supply vendors generally supply law enforcement agencies with their sexual assault kits. The contents of these kits can vary by agency. An itemized list of collected items should be submitted with the kit.

Boxes and envelopes of uniform size make storage and retrieval efficient. Given the importance of biological evidence in these cases, sexual assault kits are often retained for decades and must be stored in a manner that prevents degradation and facilitates easy retrieval and identification. Depending on the contents of the kits, a temperature- and humidity-controlled facility may be appropriate.

Extracted DNA.   Preservation of genomic DNA extracted from biological evidence is an important consideration for any handling, storage, and retrieval procedures, as this DNA may be the only source of material for future testing. Historically, extracted DNA has been stored in a preservative and then frozen or refrigerated. The stability and recovery of DNA extracts is dependent on the quantity and quality of the extracted DNA prior to storage as well as the type of tube used for storage.  However, maintaining freezers and refrigerators is costly, which has led to research on room temperature storage of DNA extracts.

Tissue Samples.   At times, preservation of tissue samples for the long term may be handled by a property and evidence custodian after the tissue has been sampled and analyzed by a crime laboratory or medical examiner. Tissue samples that are submitted for DNA analysis are usually stored at -20 °C as rapidly as possible to halt the degradation process. In cases of mass casualty disasters, freezing or refrigeration may not be immediately available. The use of preservation reagents used to stabilize tissue samples temporarily at room temperature may be advantageous.

Environmental Conditions

The proper drying and packaging of biological evidence is the first step toward achieving optimal preservation. The next step is storing it in the proper environmental conditions. Biological evidence must be stored in a fashion that not only safeguards its integrity but also ensures its protection from degradation. The storage of biological evidence may include, but is not limited to, the use of temperature- and humidity-controlled areas or freezers and refrigerators. In all cases, it should be understood that conditions of storage should include protection from moisture, excessive heat, and protection from sunlight.

Biological evidence should be stored in one of the following conditions, depending on the type of evidence, and if known, the type of analysis that will be conducted:  

  • frozen: temperature is maintained thermostatically at or below –10 °C (14 °F)  
  • refrigerated: temperature is maintained thermostatically between 2 °C and 8 °C (36 °F and 46 °F) with less than 25 % humidity  
  • temperature controlled: temperature is maintained thermostatically between 15.5 °C and 24 °C (60 °F to 75 °F) with less than 60 % humidity  
  • room temperature: temperature is equal to the ambient temperature of its surroundings; storage area may lack temperature and humidity control methods

Because of the nature of the evidence storage and management process, it is necessary to distinguish temporary storage from long-term storage. In many cases, evidence is stored temporarily because the facility handling it does not have the proper conditions to ensure its integrity for a long time. Temporary storage spaces include medical facilities and hospitals, small property rooms at law enforcement headquarters, or vehicles that transport evidence from the crime scene to long-term evidence management facilities. Temporary storage is generally defined to include any location where evidence may be stored for 72 hours or less. Long-term storage is defined as any location where evidence may be stored for more than 72 hours.


References and Further Reading

Technical Working Group on Biological Evidence Preservation (2013).  The Biological Evidence Preservation Handbook: Best Practices for Evidence Handlers.  Available:

HERC. (2012).    Regulated Medical Waste—Overview.  Available:



Modification History

File Created:  05/02/2019

Last Modified:  04/30/2021

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This work is licensed under an Open Educational Resource-Quality Master Source (OER-QMS) License.

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