Chapter 11: microbiological risk of cryopreservation of donations
Updated 21 May 2024
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Tissues, cells and gametes may be subject to long-term cryopreservation prior to use.
In the past, storage in the fluid phase of liquid nitrogen has afforded the opportunity for pathogenic contaminants to gain ingress to the stored materials. In one unit, this led to a series of hepatitis B virus (HBV) infections in recipients whose bone marrow autografts were stored in a contaminated tank. As a result of this a series of recommendations for cryopreservation were made. These apply to the storage of tissues retrieved for use as allografts by third parties as well as tissues stored for subsequent use as autografts.
It is appropriate to reduce the contamination risk by routinely storing donations in the vapour rather than the liquid phase of nitrogen, and considerable validation of this has been carried out by NHS Blood and Transplant (NHSBT). Where this is not considered possible, donations should be stored in a primary container into which access by liquid nitrogen is prevented. A secondary container should enclose the primary container, further reducing the likelihood of liquid nitrogen washing material in and out of the primary container. Once validated, such a process can be deemed to isolate an unscreened sample within the storage tank and complies with theneed for quarantine before test results are available. This may be desirable but prove not to be possible for very delicate tissues, such as human embryos and human embryonic stem cells (hESCs) cryopreserved by vitrification and stored in open pulled straws, although sealed straws and closed systems are preferable.
In view of the recognised potential for contamination of material with adventitious agents, local risk assessments must be used to guide best practice. Contamination in this way may become a concern for the repeated cryopreservation of derived cell lines. Where it is not possible to store donations in a manner which prevents contamination, it is advisable to have separate storage facilities for each known infectious risk. Storage tanks should be cleaned and decontaminated in the event of thawing. If it is found that an infectious unit has inadvertently been placed in the routine storage facility, a risk assessment should be undertaken to define any potential hazard for recipients of materials who have received material from the tank or may receive material in the future.
To preserve the integrity of storage tanks in use, only donations known to have come from uninfected donors should be placed in the tank. If the timing of procedures is such that cryopreservation is required before the results of microbiological testing can be made available, the use of a holding tank should be considered in which cryopreservation can be performed before long-term storage. This applies unless the conditions of storage can be expected to prevent the possibility of contamination.
Donors of materials intended for cryopreservation should be screened as for donors of tissue, cells or gametes.
Quarantine of samples until testing results are available is a requirement of the Human Tissue (Quality and Safety for Human Application) Regulations 2007 and subsequent Human Tissue Authority (HTA) directions.
Where automated platforms are used for cryopreservation, donations identified to come from infected donors should be removed after cryopreservation to a separate long-term storage facility. The geographical separation from the routine storage facility to a secure site for infectious donations which cannot be discarded - for example, an autologous bone marrow or a semen donation from a person about to undergo radiotherapy - removes the possibility of a third party being put at infection risk from mislabelllng or the use in error of a wrong donation. It also reduces possible contamination of the storage facility.