Long-term DNA Storage
Written by Kerstin Hammarberg   

Guidelines for long-term DNA storage

A CRIME IS COMMITTED and possible DNA is left behind by a suspect. The crime-scene technicians collect samples, package them, and bring them to the property-and-evidence (P&E) unit for storage. Eventually, the samples make their way to a crime lab for testing and analysis. Upon their return to the P&E unit, how should they continue to be stored? How long do they need to be kept?


The results of a recent study by the National Institute of Justice (NIJ), the DNA Field Experiment, showed that in property-crime cases where DNA evidence is processed, twice as many suspects are identified, twice as many suspects are arrested, and more than twice as many go to court. Considering the results of studies such as this, the heightened expectations of jurors, and the continued success of DNA evidence in identifying criminals, biological specimens are becoming increasingly important in solving crimes. That means more departments are using this science to solve crimes—which is great for victims and the community at large!

But there are challenges that come along with that success. Blood kits, urine kits, sexual-assault kits, DNA-collection kits, and other biological specimens take up a growing amount of space in P&E rooms. Knowing that space limitations are a constant issue for any property-and-evidence unit, how can we best store things over the long term in a way that allows future testing and analysis to be done effectively?

These are questions P&E units all over the country are asking. As technology and science continue to improve, biological specimens and samples are becoming more important in proving a case in the judicial process. But where do we put these samples? How do we keep them in a condition that allows them to be tested at a later date?

After asking these questions at local crime labs, at the Minnesota Bureau of Criminal Apprehension (BCA), at other local police departments, at the FBI Crime Laboratory, and after checking with the American Society of Crime Lab Directors (ASCLD), the International Association for Property and Evidence (IAPE), and other evidence-retention specialists, there is no clear answer. Each organization suggests talking to the state BCA or local crime lab for advice, but no one organization seems to have guidelines that are clear and consistent. Each one looks to the other for the answers.

I did some research into this question to meet one objective:

To provide guidance for the storage and long-term retention of biological evidence taken into custody of the police department. The goal is to create a best-practices model that can be used by local law enforcement for the long-term retention of biological evidence.

Before analysis in the lab, keep
biological evidence in cold storage

While doing my research, I discovered that the best method of storage from the time of collection to initial analysis is cold storage—refrigeration or freezing. After samples have been tested or analyzed and returned to the local P&E unit, most items may be stored long-term at a constant room temperature. Items such as tissue, bones, and teeth should be kept frozen regardless of their testing and analysis status.

The difference between refrigeration and freezing is based on what type of evidence is being stored. Typically, slides, swabs, and other types of “smudge” evidence should be frozen until thoroughly tested and analyzed. Evidence (particularly liquid evidence) that is stored in tubes or containers should be refrigerated. This is due to the fact that the material in the container may expand when frozen, potentially breaking the container.

After testing in the lab,
may be the best solution

After testing and analysis, items can be moved to room-temperature storage. In my research, I learned that it is not necessary to maintain items in cold storage after the process of testing and analysis, because laboratory personnel should be documenting in written reports the results of testing that are relevant to the case. If further future testing is necessary, a baseline is already established and other samples can be taken as necessary for comparison and future analysis.

There is also an issue of maintaining the evidence in its original condition. In most refrigerators, there is a challenge with humidity regulation. If evidence is retained in cold storage, you must employ a method of preventing the formation of mold and mildew on the boxes. Again, this issue can be better addressed as you move the specimens from cold storage to general room-temperature storage after testing and analysis has been performed.

In drying evidence such as clothing, it is important to make sure the item is completely dry prior to storage. One of the biggest challenges is preventing the degradation of the evidence by mold or mildew. Departments should have some method of thoroughly drying items, whether it is in a commercially developed drying system or a specifically designed drying area.

After thoroughly drying the clothing, it should always be packaged in paper. This is due to the need to maintain airflow and prevent the growth of mold or mildew that can destroy biological evidence. Plastic is always a bad idea, as it encourages mold and mildew growth.

Most biological evidence is tested or analyzed within three years. After three years, if the items have not been tested, they should be moved to room-temperature storage. Kept at a relatively constant temperature, samples will remain in a state that will allow testing and analysis to occur at any point in the future.

Moving evidence to general, room-temperature storage solves part of the challenge of an ever-increasing need for cold storage. By storing evidence on open shelving at room temperature, the need to purchase more cold storage decreases. Departments then can main-tain existing cold-storage facilities, knowing that after testing and analysis, the specimens can be stored in the general-storage areas.

Looking ahead to find solutions
to future challenges

As our department moves cautiously forward with this new method of storage, we are working with the local court system to educate and inform them of the challenges—and the potential solutions—that are involved with long-term storage of biological-specimen evidence. As the scientific technology advances in the field of biological specimens, there may be more specimens collected that will lead to an ever-increasing need for space.

The reality about any evidence storage for biological specimens is that over time—regardless of the method of storage chosen—there will be some degree of sample degradation. As tech-nology and science continue to make advances, better methods of long-term storage need to be developed. For those of us who work in the field of P&E-unit management, those better methods will be a welcome solution to an ever-increasing challenge.

Use caution when establishing
your own agency’s procedures

The information provided in this article is the result of careful research and communication with a number of local and national authorities on long-term DNA storage. It is always important for any agency to work closely with its own laboratory and court system to determine the best methods of storing this important evidence.

Another good source of information
for long-term DNA storage

In 2001, California passed the Post-conviction DNA Testing Law (SB 1342) that stated “...it is the responsibility of governmental entities, including the courts, in felony conviction cases to retain evidence after conviction in a manner suitable for DNA testing.” A report submitted by the Postconviction DNA Testing Task Force is the best source of information that I located that clearly outlines recommendations for long-term storage of biological specimens. You can download a PDF of the Task Force report at: http://ag.ca.gov/publications/finalproof.pdf

About the Author

The author, Kerstin Hammarberg, is a Certified Property and Evidence Specialist through the International Association for Property and Evidence (IAPE). She has worked in the field of evidence management in Minneapolis, Minnesota and surrounding suburbs for 12 years, the last eight years with Minneapolis Police as the Property and Evidence Unit Supervisor. She can be reached at: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

"Guidelines for Long-Term DNA Storage", written by Kerstin Hammarberg
November-December 2008 (Volume 6, Number 6)
Evidence Technology Magazine
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