Infection Control Today - 06/2003: Getting Wrapped Up in Packaging Choices

June 1, 2003

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The Facts: Wet Packs and Plastic Accessory

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

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,

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

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
  • 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
  • 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.
    M. Pyrek