From:Date: 28 November 2001, 17:30 [UTC] Subject: Re: Sterile instrument sets shelf life
You have asked a very good question. The purpose of a shelf life policy is to protect the patient from unsafe products. Some items, such as some types of suture, degrade naturally over time and have a shelf life that is based the rate of decay. Instrument sets containing metal instruments are a different class of device because under normal circumstances one does not expect the instruments to decay. The primary concern with instrument sets is the sterility of the contents. The wrapper or container protect the instruments inside from microbial contamination. Why does the wrapper or container protect the instruments on one day, but not on the following day? Why does it fail? Can the microbes read the date and know to stay out until after the expiration date?
In 1982 I took over a storeroom that had previously belonged to the Surgery Department. (University of Iowa Hospitals and Clinics) I found sets of surgical instruments from 1945 wrapped with newspaper sitting on the shelves. These sets had over one quarter inch or one centimeter of dust sitting on top of them. On a whim, I brought one of the sets to the Clinical Microbiology lab. We opened the set and they tested the instruments. They were still sterile after sitting undisturbed on the shelf for 37 years! Why didn't the old brittle newspaper fail as a protector of sterile instruments?
It turns out that the answer has to do with the handling of the wrapper or container. These sets had sat undisturbed since the day they were sterilized. When the sterile device is sitting undisturbed, the wrapper is not challenged and the device remains sterile. Handling of the set is when the wrapper may be compromised. Imagine a magnified image of a muslin wrapper. Between the threads are holes. When the wrapper is squeezed and then released, air is forced in and out of the package which can blow micro-organisms into the package. If the wrapper gets wet, micro-organisms can swim right through the paper or cloth wrappers. Dragging a set from a shelf instead of lifting the set from the shelf can force micro-organisms through a wrapper. If a heavy package is dropped, then micro-organisms can be forced through the wrapper. (Experiments have shown this rarely happens with peel-packaged items under 100 grams dropped from table height.)
The types of events listed above create the potential for the wrapper or container to fail. In the past it was believed the safest policy was to assume every item processed by the sterilization team would be handled a minimum of three to five times a week. With the low quality of protection offered by the standard muslin wrappers, this meant an item should be considered as potentially contaminated after a set time - 1 month, 3 months, 6 months; whatever the hospital Infection Control department felt comfortable with. Today, many hospitals have decided to use what is called "Event Related Sterility". This involves a large investment in educating every person in the hospital in understanding what type of events can potentially compromise the wrapper. They also have to know to send the item back for reprocessing! This educating of the staff is easier to do today because the quality of the available wrappers is better than the cloth wrappers used in the past. The modern wrappers work better at protecting items, so the level of control can be relaxed a little. Even better are container systems. With rigid instrument containers, there is almost zero chance of a tear or a hole in the wrapper because the filters are well protected! This is a giant step forward compared to putting a piece of paper or muslin cloth around a set. The return on the investment is that no items are reprocessed unless something happened to the item to potentially compromise the wrapper. With this system, the items are labeled with the load number and sterilizer ID number and the sterilization date. No expiration date is given. I believe a wise material manager will still want to do periodic date checks of items; not for sterility concerns, but to know if the investment is being used well. One of many ways to do this can be to use a one year expiration date and have the staff report all expired items to the manager. A set or pack that sits for a whole year without being used might be an underutilized investment. Perhaps tearing the set down and using the components in frequently used sets might be a better option than having the item remain where it is.
I hope this sheds a little light on the mystery of expiration dating in hospitals.
Regards, Pete Bobb
(Germany) Date: 4 April 2003, 13:14 [UTC] Subject: Re: Sterile instrument sets shelf life
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...
Dipl.-Ing. (FH) Peter Wagner