Infection Control Today - 07/2003: The Facts

hinsidecen.gif (3062 bytes)

The Facts: Wet Packs and Plastic Accessory Cases

By Rose Seavey, RN, MBA, CNOR, ACSP, and An Nuyttens, MSc

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

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

As is well known in the industry, numerous variables influence the probability of experiencing wet packs. This paper demonstrates that wet pack issues are not specifically relevant to plastic accessory (instrument) cases. Wet pack issues may be avoided through appropriate loading, preparation, sterilization and transportation according to good hospital practices.


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

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

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

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 [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, similar information is unavailable for the parameters concerning case drying times.

Factors Contributing to Wet Pack

Numerous parameters influence the probability of experiencing wet pack. Although the following list is by no means exhaustive, it does 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 at independent testing facilities. The first study1 cited provides extensive data on drying cycles. This study sought to determine if significant drying time differences existed among plastic, hybrid and metal cases.

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

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

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

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

A second comparative study2 on wet pack issues was 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 were chosen (e.g., 275 degrees F or 134 degrees C) during 18 minutes of exposure. Drying time was 20 minutes. Inspection and weight measurement were evaluated immediately after opening the autoclave.

Three different kinds of cases were evaluated:

one plastic case and two different types of hybrid cases. The first hybrid case consisted of a stainless steel base and plastic lid. The second hybrid case consisted of a plastic and metal base with a plastic lid. Both a visual inspection and a weight measurement evaluation were completed 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 trays without the instruments. Column 4 indicates the weight of the instruments. Column 5 presents the total weight of the pack. All packs weighed less than 10 kilograms, 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 tested for wet pack issues, including plastic cases, metal cases, and hybrid systems (combination metal and plastics). When applying good hospital practices, the results demonstrate that all cases, including plastic cases, performed well and did not exhibit wet pack issues.

It is the responsibility of each healthcare facility to consistently follow the medical device manufacturers recommendations concerning care, cleaning and sterilization. Each healthcare facility 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 director of the sterile processing department at the Childrens Hospital in Denver. She is the current president of the American Society of Healthcare Central Service Professionals (ASHCSP). An Nuyttens, MSc, is a global market manager for Solvay Advanced Polymers, LLC, a high-performance plastics manufacturer in Alpharetta, Ga.

Sterilization: An Engineering Perspective

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

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

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

McLean says the air conditioning system should be dedicated to the area in which the sterilizing processing facility is located. He adds that the ventilation requires the air device to draw air from the outside, passing it through the sterile area and then into the exhaust system so that 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

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.