Infection Control Today - 07/2003: The Facts


The Facts: Wet Packs and Plastic AccessoryCases

By Rose Seavey, RN, MBA, CNOR, ACSP, and AnNuyttens, MSc

In the sterile processing community, there is aperception that plastic accessory (instrument) cases have a greater tendency toexperience wet pack problems. This article addresses wrapped accessory(instrument) cases and is a result of independent studies.

A wet pack problem exists if moisture isexhibited upon completion of a sterilization cycle and appropriate cool-downcycle. Visible moisture can provide a path for microorganisms to enter andcontaminate a wrapped pack. Polyphenylsulfone is the principal high-performancepolymer used to manufacture all plastic accessory cases. It is also thestructural plastic component in hybrid systems (cases composed of both metal andplastic materials).

As is well known in the industry, numerousvariables influence the probability of experiencing wet packs. This paperdemonstrates that wet pack issues are not specifically relevant to plasticaccessory (instrument) cases. Wet pack issues may be avoided through appropriateloading, preparation, sterilization and transportation according to goodhospital practices.


Three types of accessory cases were studied:plastic, metal and hybrid (combination metal and plastic) cases. All packs wereprepared, loaded and sterilized in accordance to the Association for theAdvancement of Medical Instrumentation (AAMI) and the European Committee forStandardization (CEN) standards.

The studies were performed at independentinstitutes in the U.S.A. and Europe and focused on steam sterilization.

Several parameters were evaluated including theloading, weight, mass and different case designs.

From a regulatory perspective, the wet pack issueis complicated by the lack of a generally accepted quantitative definition ortest method for determining when a pack is considered a wet pack. Standards forevaluating sterilization efficacy have been established by several associationsin the United States [e.g., AAMI, Association of periOperative Registered Nurses(AORN), American Society for Healthcare Central Services Professionals (ASHCSP),the Centers for Disease Control and Prevention (CDC)] and in Europe [e.g., CEN,Deutsche Industrie Norm (DIN) and Norme Francaise (NF)]. Unfortunately, similarinformation is unavailable for the parameters concerning case drying times.

Factors Contributing to Wet Pack

Numerous parameters influence the probability ofexperiencing wet pack. Although the following list is by no means exhaustive, itdoes serve to demonstrate the complexity of the wet pack issue.

  • Steam quality

  • Pack preparation and handling

  • Types and sizes of sterilizers

  • Sterilizer cycles and drying times

  • Location of pack in the sterilizer

  • Types of load (textile vs. instrument sets - total weight of the pack)

  • Wraps: sizes and types (water permeable vs. water impermeable)

  • Design of cases and trays

  • Sterilizer loading techniques (stacking of packs)

Test Conditions and Results

Several studies on wet packs were performed atindependent testing facilities. The first study1 cited provides extensive dataon drying cycles. This study sought to determine if significant drying timedifferences existed among plastic, hybrid and metal cases.

A wide variety of cases and loadingconfigurations were tested, with each wrapped case weighing from two to 28pounds. This heavy loading represents extreme conditions under which a wet packproblem is more likely to occur. The test also included stacking of cases, whichalso meaningfully increases the likelihood of a wet pack problem. All packs weredouble wrapped in permeable sterilization wraps, which are Food and DrugAdministration (FDA) cleared.

The gravity cycle consisted of a 30-minuteexposure at 270 degrees Fahrenheit (132 degrees Celsius) and 20 minutes ofdrying time in a standard steam sterilizer with a 39- inch by 26-inch by 26-inchchamber. All packs were evaluated using a visual inspection process and wereweighed immediately after opening the autoclave. Although weighing before andafter sterilizing does not reflect field procedures, this was done to evaluatethe amount of water absorbed or remaining in the pack. This method of weighingthe pack does reflect possible, upcoming regulatory changes.

The test was repeated three times. Table 1summarizes the test results.

After 20 minutes of drying, no significantdifference was detected between the plastic, hybrid and metal cases relative towet packs. These results show the capability to meet the ANSI/AAMI ST 46:2002Standard. This standard recommends a drying time of at least 30 minutes followedby a 30-minute cooling time.

A second comparative study2 on wet pack issueswas performed following current European Standards (NF EN 285 and NF EN 868-8).These tests were completed at a well-recognized hospital. Pre-vacuum cycles werechosen (e.g., 275 degrees F or 134 degrees C) during 18 minutes of exposure.Drying time was 20 minutes. Inspection and weight measurement were evaluatedimmediately after opening the autoclave.

Three different kinds of cases were evaluated:

one plastic case and two different types ofhybrid cases. The first hybrid case consisted of a stainless steel base andplastic lid. The second hybrid case consisted of a plastic and metal base with aplastic lid. Both a visual inspection and a weight measurement evaluation werecompleted on the packs. All packs were double wrapped with FDA cleared,non-woven sterilization wraps.

Table 2 shows the results of those tests.

Column 3 shows the weight of the case and trayswithout the instruments. Column 4 indicates the weight of the instruments.Column 5 presents the total weight of the pack. All packs weighed less than 10kilograms, as authorized by European regulatory organizations. As can be seen,all three types of packs performed well and no wet pack problems were observed.


A wide variety of delivery systems were testedfor wet pack issues, including plastic cases, metal cases, and hybrid systems(combination metal and plastics). When applying good hospital practices, theresults demonstrate that all cases, including plastic cases, performed well anddid not exhibit wet pack issues.

It is the responsibility of each healthcarefacility to consistently follow the medical device manufacturersrecommendations concerning care, cleaning and sterilization. Each healthcarefacility should also follow the recommendations given in ANSI/AAMI ST46: 2002 -Steam Sterilization and Sterility Assurance in Healthcare Facilities.

Rose Seavey, RN, MBA, CNOR, ACSP, is directorof the sterile processing department at the Childrens Hospital in Denver. Sheis the current president of the American Society of Healthcare Central ServiceProfessionals (ASHCSP). An Nuyttens, MSc, is a global market manager for SolvayAdvanced Polymers, LLC, a high-performance plastics manufacturer in Alpharetta,Ga.

Sterilization: An Engineering Perspective

Can sterilization and construction have anythingin common? Perhaps more than you think. As you move from Inside Central Sterileto the next pages feature on construction-related infection control issues,take a moment to ponder engineerings viewpoint on low-temperaturesterilization.

Ross McLean, an engineer for an Australianhealthcare system and a recent presenter at the International Federation ofHospital Engineering Congress, emphasizes that consideration should be given towork flow, wall and floor surface finishes that are constructed in materialssuitable to the sterile environment of a sterile processing department (SPD).

Because surface irregularities can harbormicroorganisms, the walls, floors and surfaces in an SPD should be constructedso that seams are sealed and difficult-to-clean corners are minimized, and thatthey are non-porous, smooth and capable of being easily cleaned.

McLean says the air conditioning system should bededicated to the area in which the sterilizing processing facility is located.He adds that the ventilation requires the air device to draw air from theoutside, passing it through the sterile area and then into the exhaust system sothat no vapor can escape into the SPD. He recommends a laminar pattern.

Other points to consider:

  • Any vapor should be captured as close to the source of generation as possible. A fume hood or other safety device should contain the lowtemperature sterilization product and remove all vapors produced

  • The low-temperature sterilization facility should be located in a low-traffic area and free from interference from other air currents

  • The point of discharge should be as far away as possible from open windows or other fresh-air intakes of the healthcare facility

  • Efficient ventilation (e.g., minimum 10 air changes per hour with pressure negative to the sterilization area) should be maintained When it comes to water quality, McLean says an SPD should have a clean, continuous water supply, with attention paid to the suitability of the water supply in the cleaning and subsequent drying of instruments.

Other considerations include:

  • Cold water should be delivered to the SPD via a backflow prevention device to ensure no possible contamination of the hospitals cold water supply

  • Separate sinks suitable for the disposal of liquid wastes, for cleaning and for handwashing must be provided McLean says sterilizers and associated equipment should be maintained regularly, and the healthcare facility should seek a preventive maintenance program from the manufacturer. Cleaning and maintenance of equipment is essential to ensure that the equipment functions correctly, risks of cross contamination are kept to a minimum and a clean environment is maintained, he says. Kelly M. Pyrek

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