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Shelf life - The problem of maintenance of sterility.......
To be able to judge the capability of a sterile pack to maintain sterility, it was for years common practice to put several packs "on shelf" and then investigate them after certain periods of time for microbial penetration.
Reason for that was mainly that information was needed to answer the question,
- whether the (porous) packaging material might deteriorate during longer storage
- whether extended storage alone might already cause microbial penetration into a pack
Today, shelf life tests are no longer regarded to "meet the point":
- Material deterioration: Manufacturers designed packaging materials (modern nonwoven) and -concepts (rigid containers) which do not deteriorate by time, such do no longer need to perform extensive shelf life testing.
Note: Extended sterilization holding times (due to i.e. the BSE / mad cow (Creutzfeld Jacob) syndrom) introduce new risks: some packaging materials may be deteriorated by prolongued exposure to moist heat of 134 °C for >15 minutes... Such a application must be carefully validated.
- Extended storage testing: As the storage conditions for such a test are not standardized, the results are not "transferable" to another but the test environment, in so far not of value for the potential user (who has other environmental conditions at his hospital - and more important: in most of the cases shelving times of < 1 week).
Extended storage testing might even produce a false sense of security: once sterility has been attained and a set sits on the shelf, there is virtually no means during storage by which microorganisms can penetrate into the sterile pack: microorganisms cannot move by themselves, but require "events" such as the forced passage of air through the pack, penetration by liquids or vibration; all of which would not occur during storage, but might occur during handling procedures...
(Such "handling" includes, for example, re-arrangement on the shelf and transport to the operating room, when the contaminated microbiological barrier may be suddenly challenged by a variety of events (vibrations, droplets, etc.) and handling risks (tearing, puncture).
But exactly this risk - the handling of packs - is not addressed by "shelf life" testing, because in order to be able to investigate for microbial penetration during storage, the testpacks have to be handled under "laboratory conditions" (opened very carefully - under laminar flow clean room environment) in order to avoid contamination during opening (which would falsely indicate a "penetration" during shelving): such a opening technique does not consider "real practice" handling...
Limitation of storage time to reduce outside pollution
and allow aseptic opening...
Another point is, that there can be no doubt that the external surface contamination of any stored pack will gradually increase over time as a function of exposure during storage. The absolute level of external contamination will depend of the on site storage conditions and the duration of exposure.
This external contamination does, as already indicated, not represent a significant risk while the pack is "sitting on shelf", but this external contamination introduces an increased risk of a breach of sterility when packs require to be HANDLED and, last not least, when they will be OPENED: Tearing open a pack or handling the lid of a sterilization container will spread particles from the contaminated outer surface into the air of the room (= generation of contamination). These disseminated particles may then contaminate the now exposed, no longer protected product.
The level of risk due to external contamination may be rated from "low" to "high" depending on the duration and conditions of storage. In 1986, German hygienists therefore developed a "ranking table" based on the DIN 58 953 series of standards (for pouches, wrapping materials, containers) which recommends a limitation of shelfing time depending on the kind and duration of storage or alternatively the use of double barrier layers (double wraps)
A single layer packaging, regardless of the type of packaging, is according to DIN standards of 1987 not suitable for "storage" when stored "unprotected" (i.e. without clean air-conditioning or not in a closed cupboard) and should be used a.s.a.p.
The same pack can be considered "safe" when stored "protected" (i.e. in a good air-conditioning room or dust-proof cupboard). The difference is coming only from the reduced OUTER bioburden on the primary sterile pack.
If a double layer is used, the risk of contamination during opening is reduced, because the first layer (with its external contamination) can be removed as a preliminary step. The exposed very clean and nearly sterile surface of the inner layer then no longer represents such a significant risk when opened in a different place.
Summary: The risk to compromise sterility is today viewed less as an inevitable consequence of storage (shelf life) itself but more influenced by single harmful "events" which stress the sterile pack. Ranking packaging concepts by comparisom of "shelf life test data" may therefore not meet the point but might even introduce a false sense of security.
Storage conditions and duration should however be considered to be able to address the need to allow aseptic opening of packs. This means: good resp. "dust protected" storage or short storage only or use of multiple packaging layers...
Best regards,
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Dipl.-Ing. (FH) Peter Wagner
Wagner GmbH Fabrik fuer medizinische Geräte
Schulstr. 16a - D-80634 Muenchen
WEB: http://www.wagner-steriset.de
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