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 agreater tendency to experience wet pack problems. This article addresses dryingissues with wrapped instrument case and tray sets made out of plastic, metal ora combination of both.

This paper is the result of a productive and successfulcollaboration between the industry (suppliers) and the user (hospitals). Theauthors of this paper can highly recommend such collaborative initiatives asthey help both parties to understand better each others needs and wants inthe different environments.

Reusable case and tray sets are sterilization containmentdevices that need a wrap or have to be enclosed in a rigid container to maintainsterility during transport. Case and tray sets are mostly customized fororganizing an instrument set. Although less common, generic cases and trays arealso 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 principalhigh-performance plastic used to manufacture plastic case and tray sets and isalso used for the structural plastic components in hybrid systems (case and traysets composed of both metal and plastic). This particular material is used because it is the onlycommercially available plastic able to withstand thousands of autoclave cyclesand maintain its impact strength and toughness. Metal case and tray sets are mainly made out of anodizedaluminum, although stainless steel type 304 or 316 is also used when increasedchemical resistance is needed.

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

This article publishes the results of drying tests performedin both an independent laboratory and the central processing department of awell-known hospital. Several configurations have been evaluated: differentwraps, use of an absorbent towel, customized vs. generic trays, and differentdrying times.

This article is a sequel to an earlier article published inthis magazine (The Facts: Wet Packs and Plastic Accessory Cases, July 2003 ICT). The firstarticle showed the results of drying tests performed on a wide variety ofdelivery systems, including plastic case and tray sets, metal case and traysets, and hybrid systems (combination of metal and plastics). The paperconcluded that when applying good hospital practices, all sets, includingplastic 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 thecase and tray upon completion of a sterilization cycle and appropriate cool-downcycle. Visible moisture may provide a path for microorganisms to enter andcontaminate a wrapped pack.

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

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

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

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



Drying tests were performed at Northview Laboratories, Inc. inIllinois. Three types of case and tray sets were studied: plastic, metal, andhybrid (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 ofsurgical instruments. None of the trays were organizing trays with specificlocations for instruments. Each set was similarly loaded with miscellaneousmetal surgical tools and instruments to a total pack weight of approximately 20pounds. The instruments were laid directly on the tray and on the bottom part ofthe cases without the use of a mat or towel. After loading, each set wasdouble-wrapped in heavy duty blue non-woven3 wrap cleared by the Food and DrugAdministration (FDA). The steam sterilizer was a Hotpack. Six packs could beloaded in the sterilization equipment: three on the top shelf and three on thebottom shelf of the sterilization rack.

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

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

Test Results

None of the packs showed any weight gain when directly weighedafter the autoclaving cycle or after the cool-down period. Furthermore, therewas 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 contentsexamined.

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


All case and tray sets dried equally with a 15-minute dryingtime. 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, similardrying tests with the same generic case and tray sets were performed in thesterile processing department of Saint Barnabas Medical Center in Livingston,N.J.

As for the lab study, the case and tray sets were composed ofplastic, 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 andthe weight ranged from 19.9 pounds to 20.2 pounds. None of the trays were organizing trays with specificlocations for the instruments. The various instruments were randomly placedinside the trays and distributed on both levels. It should be noted that generictrays may not be the most commonly used in hospital environments. The instruments may have moved around when handled, whichcould influence test results.

All of the test cycles were performed in a Getinge USAprevacuum 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 testingbegan. This equipment is not a recent or newer model and was purposely chosen toreflect common conditions in a clinical environment.

Exposure time in the sterilizer was four minutes at 270degrees F (132C). Drying times varied depending on the test condition. Each testcondition will be discussed in greater detail in the next section. The packswere 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 commonpractice in many healthcare facilities. A cool down time of 30 minutes wasapplied after removing the packs from the sterilizer. The packs were weighedbefore each autoclave cycle, after the cycle was finished and the pack removedfrom 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 exceptfor a 20-minute drying time in the hospital versus a 15-minute drying time inthe lab. Commonly used drying times for similar case and tray sets are minimal30 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 isnot commonly used for heavy sets in clinical situations. With the lab wrap and a20- minute drying time, all packs dried and no wet packs were found.

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

The third cycle tested the drying of the case and tray setswrapped in the heavy-duty wrap; however, absorbent surgical towels were placed on the tray andthe 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 andsecond 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 setafter 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 packweight 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 setswere tested from three major orthopedic companies: two plastic, two hybrid andtwo metal. The weights of the packs varied from 19.1 pounds to 20.4 pounds. Themetal sets were single layer, the plastic sets were either two or three layersinside the case and the hybrid trays were two layers inside the case.

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

All sets were wrapped with one piece of 45-inch-squareheavy-duty Simul-Wrap. The positions of the sets were rotated in the sterilizerso that all trays were studied on top or bottom shelves and in the front, middleand back of the sterilizer. The total number of cycles was six, which means thatthe total number of packs going through the sterilization and drying cycle was36. 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 oneexception. One of the plastic case and trays sets containing three layersexhibited moisture at the outside of the pack. However, this result could not bereplicated during the five following cycles, and the same pack showed dry aftereach of these cycles. Although the single incident of wet pack with the set withthree layers could not be repeated, it is understandable that a three-layerstructure (made out either plastic, hybrid or metal) provides more challenge todry than a one-layer or two-layer case.


All customized cases dried in the standard hospitalsterilization cycle with a 30-minute drying time. No difference in dryingbehavior 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 packissues, including all plastic case and tray sets, all metal case and tray sets,and hybrid systems (combination of metal and plastics). Both generic andcustomized case and tray sets were evaluated.

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

The same generic cases and trays were evaluated in the sterileprocessing 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 of20 minutes. Also, the influence of the quality of the wrap was evaluated. Aheavy-duty wrap demonstrated the need for longer drying times than the doublenon-woven lab wrap.

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

Because generic sets (with a total load of 20 pounds) are notcommonly used in healthcare facilities, customized sets were tested. In total,six sets of about 20 pounds were tested: two metal, two hybrid and two plasticcases and trays. All customized packs filled with the specific instrument setshowed similar drying behavior with a drying time of 30 minutes resulting in drypacks.

In this study, the parameters that proved to have an influenceon drying are the density of the wraps and the design of the set. Generic setsthat are heavily loaded up to 20 pounds do show more challenges to dry thancustomized 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 SaintBarnabas Health Care System in West Orange, N.J. David Furr is with FDC Servicesin Fort Wayne, Ind. An Nuyttens is global market manager for Solvay Advanced

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