Infection Control Today - 08/2004: The Facts

The Facts:
Wet Packs and Plastic Accessory Cases

By Nancy Chobin, RN, CSPDM; David Furr; and An Nuyttens


In the sterile processing community, there is a perception that plastic reusable instrument cases and trays have a greater tendency to experience wet pack problems. This article addresses drying issues with wrapped instrument case and tray sets made out of plastic, metal or a combination of both.

This paper is the result of a productive and successful collaboration between the industry (suppliers) and the user (hospitals). The authors of this paper can highly recommend such collaborative initiatives as they help both parties to understand better each others needs and wants in the different environments.

Reusable case and tray sets are sterilization containment devices that need a wrap or have to be enclosed in a rigid container to maintain sterility during transport. Case and tray sets are mostly customized for organizing an instrument set. Although less common, generic cases and trays are also available.

Case and tray sets can be made using all plastic, all metal, or a combination of both. RADEL® R polyphenylsulfone (PPSU)1 is the principal high-performance plastic used to manufacture plastic case and tray sets and is also used for the structural plastic components in hybrid systems (case and tray sets composed of both metal and plastic). This particular material is used because it is the only commercially available plastic able to withstand thousands of autoclave cycles and maintain its impact strength and toughness. Metal case and tray sets are mainly made out of anodized aluminum, although stainless steel type 304 or 316 is also used when increased chemical resistance is needed.

Rigid sterilization containers, which use a filter to maintain a sterile barrier, are not included in this study.

This article publishes the results of drying tests performed in both an independent laboratory and the central processing department of a well-known hospital. Several configurations have been evaluated: different wraps, use of an absorbent towel, customized vs. generic trays, and different drying times.

This article is a sequel to an earlier article published in this magazine (The Facts: Wet Packs and Plastic Accessory Cases, July 2003 ICT). The first article showed the results of drying tests performed on a wide variety of delivery systems, including plastic case and tray sets, metal case and tray sets, and hybrid systems (combination of metal and plastics). The paper concluded that when applying good hospital practices, all sets, including plastic case and tray sets, performed well and did not exhibit wet pack issues.

What is a Wet Pack?

A wet pack problem exists if moisture is exhibited inside the case and tray upon completion of a sterilization cycle and appropriate cool-down cycle. Visible moisture may provide a path for microorganisms to enter and contaminate a wrapped pack.

As is well known in the industry, numerous variables influence the probability of experiencing wet packs: steam quality, types and sizes of sterilizers, pack preparation and handling, location of the pack in the sterilizer, type of the load, weight of the pack, kind of wrap and case and tray design can be considered as the most important factors.

From a regulatory perspective, the wet pack issue is complicated by the lack of a generally accepted quantitative definition or test method for determining when a pack is considered a wet pack. Standards for evaluating sterilization efficacy have been established by several associations in the United States and in Europe.

For drying requirements, there are no clear guidelines. In Europe, EN 868, Packaging materials and systems for medical devices which are to be sterilized -- Part 8 Re-usable sterilization containers discusses in Annex F how to test load drying. However, only textile packs and metal loads are considered. The European Norm describes the loading of the pack and how to weight the pack before and after the sterilization and drying cycle. The Norm allows a weight increase (or water content) as long as it is less than 0.2 percent.

The Association for the Advancement of Medical Instrumentation (AAMI) is working on a standard, Containment Devices for Reusable Medical Device Sterilization where load dryness is discussed. The draft of the guideline has been intensely scrutinized and it could take considerable additional time to finalize the guideline.



Drying tests were performed at Northview Laboratories, Inc. in Illinois. Three types of case and tray sets were studied: plastic, metal, and hybrid (combination of metal and plastic).2 In total, nine sets were tested: four plastic, three hybrid and two metal. All case and tray sets were generic, which means that none of the cases and trays were customized for a set of surgical instruments. None of the trays were organizing trays with specific locations for instruments. Each set was similarly loaded with miscellaneous metal surgical tools and instruments to a total pack weight of approximately 20 pounds. The instruments were laid directly on the tray and on the bottom part of the cases without the use of a mat or towel. After loading, each set was double-wrapped in heavy duty blue non-woven3 wrap cleared by the Food and Drug Administration (FDA). The steam sterilizer was a Hotpack. Six packs could be loaded in the sterilization equipment: three on the top shelf and three on the bottom shelf of the sterilization rack.

Three different loading patterns were run (see Table 1).

A four-minute, 270-degree F (132-degree C) pre-vacuum autoclave cycle was run using a 15-minute dry time. Each loading pattern was run three times. The lab tested nine runs in total. For every run, each case and tray set was weighed before each autoclave cycle, after the cycle was finished and again after a 30-minute cool-down period. The sets were then unwrapped and inspected for the presence of water in the case and tray.

Test Results

None of the packs showed any weight gain when directly weighed after the autoclaving cycle or after the cool-down period. Furthermore, there was no wetness on the sterilization wrap following removal from the autoclave. No water was observed inside the case once the lids were opened and the contents examined.

Table 2 shows the test results for a single run and is representative of all nine runs.

click here to view table two


All case and tray sets dried equally with a 15-minute drying time. No difference in drying behavior could be detected between the plastic, hybrid and metal cases and trays.



In order to confirm the results of the lab study, similar drying tests with the same generic case and tray sets were performed in the sterile processing department of Saint Barnabas Medical Center in Livingston, N.J.

As for the lab study, the case and tray sets were composed of plastic, metal and metal/plastic (hybrid). Of the nine sets, four were plastic, three were hybrid and two were metal. All of the loaded sets were weighed and the weight ranged from 19.9 pounds to 20.2 pounds. None of the trays were organizing trays with specific locations for the instruments. The various instruments were randomly placed inside the trays and distributed on both levels. It should be noted that generic trays may not be the most commonly used in hospital environments. The instruments may have moved around when handled, which could influence test results.

All of the test cycles were performed in a Getinge USA prevacuum steam sterilizer Model 3366. The sterilizer was 60 inches in depth. The sterilizer was installed in 1993 and had 155,528 cycles on it when testing began. This equipment is not a recent or newer model and was purposely chosen to reflect common conditions in a clinical environment.

Exposure time in the sterilizer was four minutes at 270 degrees F (132C). Drying times varied depending on the test condition. Each test condition will be discussed in greater detail in the next section. The packs were allowed to cool inside the sterilizer for 15 minutes before removing them. This differs from the lab test, but the cracking of the door is a common practice in many healthcare facilities. A cool down time of 30 minutes was applied after removing the packs from the sterilizer. The packs were weighed before each autoclave cycle, after the cycle was finished and the pack removed from the sterilizer and again after a 30-minute cool-down time.

The following conditions were tested:

  • 20-minute drying time double lab wrap (blue non-woven) no absorbent towels inside the cases and trays
  • 30-minute drying time- heavy duty Simul-Wrap4 no absorbent towels inside the cases and trays
  • 30-minute drying time heavy duty Simul-Wrap4 absorbent towels inside the cases and tray.

Test Results

The first cycle is an exact replica of the lab cycle except for a 20-minute drying time in the hospital versus a 15-minute drying time in the lab. Commonly used drying times for similar case and tray sets are minimal 30 to 45 minutes in the hospital. The sets were double- wrapped in the lab wrap (blue, non-woven) for verification purposes, although this kind of wrapper is not commonly used for heavy sets in clinical situations. With the lab wrap and a 20- minute drying time, all packs dried and no wet packs were found.

The second cycle used a heavy duty Simul- Wrap®. The drying time was 30 minutes because the heavy duty wrap presents a greater challenge to drying cases and trays. No wet packs were observed, however, two packs showed moisture between the wrapper and the case.

The third cycle tested the drying of the case and tray sets wrapped in the heavy-duty wrap; however, absorbent surgical towels were placed on the tray and the bottom of the case. All of the surgical instruments were laid on the towels, maintaining the same instrument distribution and weight as for the first and second cycle. After a 30-minute drying time and a 30-minute cool-down time, the towels were wet but none of the sets had visible moisture inside the set after drying.


The test results from the independent lab could be replicated. However, it is important to note that the use of generic trays with a total pack weight of 20 pounds may not be typical of clinical situations. Because of this, another series of tests were performed with sets of customized cases and trays.


Test Procedure

In total, six case and tray sets filled with instrument sets were tested from three major orthopedic companies: two plastic, two hybrid and two metal. The weights of the packs varied from 19.1 pounds to 20.4 pounds. The metal sets were single layer, the plastic sets were either two or three layers inside the case and the hybrid trays were two layers inside the case.

Six cycles were run in the sterile processing department of the Saint Barnabas Medical Center, using the same sterilization equipment used for the generic case and tray sets. Each cycle consisted of a four-minute exposure time and 30-minute drying time. The door to the sterilizer was opened at the end of the cycle for 15 minutes, then the autoclave cart was removed. The items on the cart were placed in a low traffic area (not located near any air conditioning ducts) and not touched for 30 minutes.

All sets were wrapped with one piece of 45-inch-square heavy-duty Simul-Wrap. The positions of the sets were rotated in the sterilizer so that all trays were studied on top or bottom shelves and in the front, middle and back of the sterilizer. The total number of cycles was six, which means that the total number of packs going through the sterilization and drying cycle was 36. This high number of packs allows verification of the results.

Test Results

For each of the configurations tested during the six cycles, all the sets were dry at the end of the sterilization cycle. There was one exception. One of the plastic case and trays sets containing three layers exhibited moisture at the outside of the pack. However, this result could not be replicated during the five following cycles, and the same pack showed dry after each of these cycles. Although the single incident of wet pack with the set with three layers could not be repeated, it is understandable that a three-layer structure (made out either plastic, hybrid or metal) provides more challenge to dry than a one-layer or two-layer case.


All customized cases dried in the standard hospital sterilization cycle with a 30-minute drying time. No difference in drying behavior could be detected between the metal (one layer inside), the plastic (two or three layers inside) and the hybrid sets (two layers inside).


A wide variety of delivery systems were tested for wet pack issues, including all plastic case and tray sets, all metal case and tray sets, and hybrid systems (combination of metal and plastics). Both generic and customized case and tray sets were evaluated.

All generic sets contained one case with a lid and one tray inside. Surgical instruments were placed on the bottom of the case and in the tray. All packs weighed about 20 pounds. First the drying behavior of generic containment devices were evaluated in a lab environment. A heavy duty lab wrap (non-woven) was used to wrap the case and tray. All generic delivery systems were completely dry when applying a drying time of 15 minutes.

The same generic cases and trays were evaluated in the sterile processing department of Saint Barnabas Medical Center. The lab results were confirmed. None of the sets wrapped in the lab wrap showed wet packs with a drying time of 20 minutes. Also, the influence of the quality of the wrap was evaluated. A heavy-duty wrap demonstrated the need for longer drying times than the double non-woven lab wrap.

The drying time needed to be extended to 30 minutes for all the generic sets, due to the greater density of the heavy-duty wrap. Absorbent towels did not allow improved drying behavior of the sets.

Because generic sets (with a total load of 20 pounds) are not commonly used in healthcare facilities, customized sets were tested. In total, six sets of about 20 pounds were tested: two metal, two hybrid and two plastic cases and trays. All customized packs filled with the specific instrument set showed similar drying behavior with a drying time of 30 minutes resulting in dry packs.

In this study, the parameters that proved to have an influence on drying are the density of the wraps and the design of the set. Generic sets that are heavily loaded up to 20 pounds do show more challenges to dry than customized sets. The tests showed that the material choice (metal vs. plastic) does not influence significantly the drying behavior.

Nancy Chobin, RN, CSPDM, is corporate SPD educator at Saint Barnabas Health Care System in West Orange, N.J. David Furr is with FDC Services in Fort Wayne, Ind. An Nuyttens is global market manager for Solvay Advanced

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