What is Sterilization?

Sterilization is the validated process that is used to render a product free from viable microorganisms.  “The type and level of microbial kill achieved depends on factors such as the type and population of microorganisms present, the type of antimicrobial agent, the concentration of the antimicrobial agent, the exposure time and the exposure temperature.” (AAMI ST79:2017, 2.70, page 8) A sterilization cycle is the “defined sequence of operational steps designed to achieve sterilization, which is carried out in a sealed chamber.” (AAMI ST79:2017, 2.117, page 10).  According to the Spaulding Classification System, sterilization is required for items that will enter sterile tissue, such as surgical instruments. There are 3 conditions necessary for sterilization: cleaning and decontamination, conditions capable of killing microorganisms, and contact with a sterilizing agent under appropriate conditions.

What is Steam Sterilization?

Steam sterilization is a “sterilization process that uses saturated steam under pressure, for a specified exposure time and at a specified temperature, as a sterilizing agent.” (AAMI ST79:2017, 2.109, page 10) Steam sterilization cycles typically used in health care facilities include the gravity-displacement cycle and two types of dynamic-air-removal cycles. One type of dynamic-air-removal cycle, the prevacuum cycle, removes air from the chamber and load by means of pressure and vacuum excursions. The other type, the steam-flush pressure-pulse (SFPP) cycle, removes air with a series of steam flushes and pressure pulses above atmospheric pressure.

It is important for sterile processing professionals to verify that the sterile barrier system and the item to be sterilized have been validated for the steam sterilization cycle to be used. The sterilization cycles used should be FDA cleared and should incorporate sterilization monitoring accessories (e.g., CIs, BIs, process challenge devices [PCDs]) and sterilization packaging labeled and cleared for that sterilization cycle.

What are the Differences between Pre-Vac and Gravity Cycles?

The two most commonly utilized steam sterilization cycles are pre-vacuum and gravity cycles.

A Pre-Vac Cycle will begin with a series of alternating steam pressure injections and vacuum draws (also called pulses) to dynamically remove the air from the chamber. Drawing a vacuum to remove ambient air from the chamber allows steam to be sucked into areas where it would otherwise have difficulty penetrating. The absence of air within the chamber allows “steam to penetrate the load almost instantaneously” resulting in more reliable sterilization and shorter sterilization cycle times.

During a Gravity Cycle, steam is pumped into a chamber containing ambient air. Because steam has a lower density than air, it rises to the top of the chamber and eventually displaces all the air. As steam fills the chamber, the air is forced out through a drain vent. By pushing the air out, the steam is able to directly contact the load and begin to sterilize it.  Sterilization cycle times for gravity loads are usually longer.

Why Steam Sterilization?

Steam Sterilization is the oldest, safest, cheapest, and best understood method of sterilization. As such, it is the most commonly used method of sterilization for surgical instruments in the surgical  setting. Steam sterilization has several advantages over other methods of sterilization:

  • Steam is non-toxic, in contrast to some of the hazardous chemicals used for sterilization. As such it does not need a chemical monitoring system to protect employees
  • Steam is readily available in healthcare facilities
  • Steam is fairly easy to control
  • Steam is consistent, reliable, and repeatable
  • Steam is fairly low-cost in comparison to other methods

Essential Conditions for Sterility - Cleaning and Decontamination

Before any sterilization process, items must be thoroughly cleaned and decontaminated.  Cleaning removes microorganisms and other organic and inorganic materials. Cleaning does not kill microorganisms.  Rinsing removes detergent and other residues that might interfere with subsequent processes. If an item is not appropriately cleaned and decontaminated, it cannot be sterilized. Appropriate cleaning for a device will vary depending on the design and complexity of the device. Instructions for appropriately cleaning and decontaminating the device will be found in the device manufacturer’s Instructions for Use (IFU).

Essential Conditions for Sterility - Killing Microorganisms

During the sterilization cycle, there should be conditions capable of killing microorganisms.  An example of these conditions include the the appropriate removal of ambient air and pressure of saturated steam. There are three IFUs that must be adhered to when preparing an item for sterilization:

  • the sterilizer for cleared sterilization parameters
  • the device manufacturer for sterilization parameters(i.e. time and temperature)
  • the packaging system for dry time and storage requirements

Essential Conditions for Sterility - Contact with Sterilizing Agent

For instruments to be sterilized, they must have direct contact with the sterilizing agent.  In a steam sterilizer, air is removed during the conditioning phase so that steam can directly contact instruments.  How a sterilizer is loaded is also an important consideration when assuring that items in the sterilizer have contact with the sterilizing agent. Instrument packaging should be positioned in such a way that the sterilant can reach all parts of the items to be sterilized. The following should be considered when loading a sterilizer for processing:

  • Written policies and procedures for loading sterilizers
  • The Sterilizer IFU, the Medical Device IFUs, and the Sterile Packaging IFU
  • The Load configuration. Heavier items should be placed on the bottom rack, weight should be distributed evenly, and appropriate space should be given between packs
  • If a shelf liner will be used, follow the liner manufacturer’s IFU for use and replacement instructions

What is Terminal Sterilization?

According to AAMI ST79:2017, terminal sterilization is the “process by which the product is sterilized within a sterile barrier system that permits storage for use at a later time.” (AAMI ST79:2017, 2.125, page 10, emphasis added). The key distinctive that separates terminal sterilization from Immediate Use Steam Sterilization (IUSS) is that the terminal sterilization process produces a sterile product that can be stored. Any instruments sterilized at a health care facility in advance for a surgery/procedure are considered terminally sterilized if the sterile barrier system has storage.

What Options Do You Have for Sterile Packaging?

The three most prominent sterile barrier systems available for steam sterilization are paper-plastic pouches, sterilization wraps, and rigid container systems. According to AAMI ST79, a sterile barrier system should:

  1. allow air removal to permit sterilant penetration of the package contents;
  2. provide a barrier to microorganisms during sterilization processing, handling, distribution, transport, and storage;
  3. resist tearing or puncture;
  4. allow a method of sealing that results in a complete seal that is tamper-evident and provides seal integrity;
  5. maintain protection for the sterile contents during storage and transportation to the point of use;
  6. allow for aseptic presentation;
  7. be free of toxic components and nonfast dyes;
  8. be non-linting; and
  9. be compatible with the intended methods of sterilization, sterilization parameters, and the devices to be sterilized.

(AAMI ST79:2017, 9.2, page 52)

Sterile Packaging - Peel Pack

Paper-Plastic pouches are generally used for single items that are small, lightweight, and low-profile. Pouches should be used, filled, sealed, and opened according to the pouch manufacturer’s written IFU. When selecting the appropriate size of pouch, the item should be able to fit inside the packaging without putting excessive pressure on the seals or package. When the pouch is closed, all the seals need to be smooth and free from folds, bubbles, or wrinkles.  When sterilizing paper-plastic pouches, the pouches should be arranged on their edge, lined up paper to plastic.

Sterile Packaging - Wraps

Sterilization wraps can be either woven wraps which are laundered between each sterilization cycle, or nonwoven wraps which are intended for only one use.  Before using a sterilization wrap, the wrap should be inspected for defects which could affect the performance of the material. Instruments sterilized in wrap should be packaged in such a way to allow for the aseptic transfer of the sterile instrument from the package to the sterile field. The envelope fold and square fold techniques for sterilization wrap are described in AAMI ST79:2017, 9.6, page 55. For sterilization parameters and handling of the wraps, the user should verify the instructions in the wrap manufacturer’s instructions for use (IFU).

Sterile Packaging - Rigid Container

Rigid containers have several advantages as a sterile barrier system, as they are resistant to punctures and are generally faster to package/close. Reusable rigid containers need to be inspected between each use to ensure that it is not damaged and is performing as it was designed. Notable examples of rigid container damage would include a loose retention plate or a dented lid that does not seal properly with the container bottom.  Rigid containers must be cleaned in between each use. Instructions for cleaning and inspecting the rigid container can be found in the manufacturer’s instructions for use (IFU).

What is Immediate Use Steam Sterilization (IUSS)?

AAMI defines Immediate Use Steam Sterilization (IUSS) as a “Sterilization method that involves the shortest possible time between a sterilized item’s removal from the sterilizer and its aseptic transfer to the sterile field. Immediacy implies that a sterilized item is used during the procedure for which it was sterilized and in a manner that minimizes its exposure to air and other environmental contaminants. A sterilized item intended for immediate use is not stored for future use nor held from one case to another. Immediacy, rather than being defined according to a specific time frame, is established through the critical analysis and expert collaboration of the health care team.” (AAMI ST79:2017, 2.50, page 7, emphasis added)

A key distinctive of IUSS is that it cannot and should not be stored for later use but is intended for immediate use.  IUSS should be kept to a minimum and should only be used when absolutely necessary in an urgent clinical situation.

Flash Sterilization - A Change in Terminology

In 2011, a Multi-Society Position Paper was released regarding the new practice of Immediate Use Steam Sterilization.  Up until this time, “Flash Sterilization” had been used to describe steam sterilization cycles where unwrapped medical instruments were sterilized in an abbreviated cycle and then used promptly after cycle completion without being stored. The term “Immediate Use Steam Sterilization” did more than just replace the term “Flash Sterilization.” The members of these professional societies believed that the term “Flash Sterilization” did not adequately describe the variety of steam sterilization cycles used to process items, the need for cleaning and decontamination steps in processing items, and the use of wraps or rigid containers to maintain the instruments’ sterility and allow the devices to be stored for later use.

The open baskets of the past were no longer to be acceptable; the item being sterilized should be prepared for sterilization according to the Manufacturer’s IFU. This position paper did more than mark a change in terminology, it shifted an industry practice.

Read the full position paper here:


The Importance of Defining Immediacy

Because the AAMI ST79:2017 definition of Immediate Use Steam Sterilization (IUSS) leaves “immediacy” to be defined through the critical analysis and collaboration of the healthcare team, it is important that each organization work to define immediacy. If a facility tells a surveyor/inspector during an infection control survey that they follow AAMI ST79, it would be fair for them to ask for the facility’s policy on immediacy. One approach to defining immediacy would be to state in a policy that immediacy involves using a sterile barrier system that cannot be stored for future use. If the sterile barrier system (rigid container, peel pack, and/or wrap) does not have the capability of being stored for future use, the record would be documented as an IUSS cycle.  This would meet the requirement of defining immediacy for the organization and would clearly define what the facility considers an IUSS cycle.

What is a Device's Shelf Life?

A device’s shelf life refers to the period of time during which a sterilized item is considered safe to use. Shelf life can refer to the period of time that the sterile packaging can keep the device sterile (sterility maintenance) or the period of time that the device can remain usable (device degradation). Before surgical devices are marketed, they must go through a 510(k) approval process with the FDA. In a 510(k), shelf life testing results will be presented to the FDA for clearance to establish a storage claim.  Following the FDA’s procedure, the shelf life of a device may be expanded by the manufacturer by performing additional shelf life testing beyond the scope of the original storage claim that was cleared as part of the 510(k).

Event Related Sterility or Expiratory Dating?

Healthcare facilities use two different practices of sterility maintenance: event-related sterility maintenance and expiratory dating.  Event-related sterility maintenance considers a package sterile so long as no “event” has happened with the item.  Event-related sterility considers factors such as packaging material, storage conditions, the methods of transport, and the amount and conditions of handling. Expiratory dating practice considers in-house sterilized goods as having a finite life, and packages are labeled with both a sterilization and expiration date. It is estimated that 90% of hospitals use event-related sterility maintenance as their practice. It is also possible in rare exceptions for a package to be both event-related have an expiration date. If an item in a tray has a finite life (such as a balloon), that tray would need an expiration date on the package matching the useful life or expiration date of that item.

Be Prepared for your Survey

Every department should be working to assure their policies and procedures meet industry guidelines and the needs of the organization. These are some important categories to consider to make sure your sterilization process is ready for a survey:

  • Orientation – New hires should have documentation that they have completed all required training.
  • Trainings – This will be an ongoing effort and can include things like vendor in-services, staff meetings, and webinars.Training is never a “one and done” event.
  • Competencies – The facility must develop meaningful competencies and periodically assess staff.These include competencies on all equipment used in the sterilization process.
  • Policies and Procedures – Each department must have policies and procedures outlining their processes.These should be reviewed at least annually, and they should always be realistic, timely, and current.
  • Documentation – Technicians should have access to manufacturer’s IFUs, Safety Data Sheets (SDS), and equipment operator’s manuals.Technicians should also be trained on how to find and interpret these documents.

Interpreting IFU's

In order to properly sterilize a surgical instrument, the processing facility must follow the manufacturer’s instructions for use.  It can become confusing when figuring out which IFU to follow.  For sterilization instructions, there are typically at least 3 IFUs to consult: the device, the sterilizer, and the packaging system. The sterilizer IFU provides information on the cleared sterilization cycles and parameters, as well as load configurations.  The device IFU gives specific sterilization parameters (time and temperature) required to sterilize the device.  The packaging system gives the dry time and storage requirements. After consulting these three IFUs, a department can confidently process the device in compliance with the manufacturers’ instructions.

Dry Times and Terminal Sterilization

Dry time is not considered part of the sterilization cycle but is a term that references sterility maintenance. Packaging systems may be  cleared by the FDA to have dry time or require no dry time.  If a device is cleared by the FDA to have retained moisture and still be able to be stored, then it has a shelf life and is not considered Immediate Use Steam Sterilization (IUSS).  Packaging systems without storage claims must be used immediately, whereas packaging systems with FDA cleared storage claims can be considered terminal sterilization.

Quality Assurance - Physical Monitors

As part of a department’s Quality Assurance program for sterilization, physical monitors should be used to monitor sterilizer performance.  Examples of physical monitoring of sterilization cycles include charts, printouts, or digital data. Technicians should use physical monitors to verify that all cycle parameters were met. Confirmation of cycle parameters should be documented by the operator by signing the chart or printout, and those results should be retained as part of the sterilization records. Physical monitoring is necessary to detect malfunctions as soon as possible so corrective actions can be taken if necessary.

Quality Assurance - Chemical Indicators

Chemical Indicators are an important part of sterilization Quality Assurance. Chemical Indicators are defined as “Devices used to monitor the presence or attainment of one or more of the parameters required for a satisfactory sterilization process or used in specific tests of sterilization equipment.” (AAMI ST79:2017, 2.10, page 4) Internal indicators should be used inside each product sterilized as a measure to help identify any potential sterilization failures.  Chemical Indicators can also be used in a process challenge device (PCD) on a sterilization load to demonstrate the efficacy of the sterilization process.  The results of the chemical indicator should be reviewed and documented as part of the routine process for releasing sterilization loads.

Quality Assurance - Biological Indicators

Biological Indicators (BIs) are an important part of sterilization Quality Assurance. Biological Indicators are another sterilization process indicator device used to help identify potential sterilization process failures.  What makes biological indicators unique is that they are the only sterilization process monitoring device that provides a direct measure of the lethality of the sterilization process.  The BI contains a known number of microorganisms, of known resistance to the mode of sterilization.  It is important to choose the correct BI for the sterilization cycle being run, and the user should follow the manufacturer’s instructions for use. BIs should be run at least weekly on every cycle type, but preferably every day the sterilizer is in use. Additionally, BIs with Type 5 integrating indicators within PCDs should be used to monitor every load containing implants.

Releasing Sterilizer Loads for Use

Every sterilization cycle should be monitored with physical monitors (i.e. printouts or data) and a chemical indicator monitor.  Loads should be released after the conclusion of the cycle, only after the physical monitor and chemical indicator have been reviewed by an experienced, knowledgeable person.  For loads containing implants, a biological indicator PCD should be included on the sterilization load.  In addition to reviewing the sterilization parameters on physical monitors and reviewing the chemical indicator, the load should be quarantined until the results of the biological indicator testing are available.

Principles for Sterile Storage

Storage conditions to maintain sterility is a very important consideration.  Environmentally, the sterile storage area should be well ventilated with a controlled temperature and humidity. These environmental conditions should be actively monitored and documented at least daily.  Traffic into and out of the sterile storage area should be kept to a minimum.  Sterile sets should be rotated in storage to ensure that the set which was sterilized first gets used first (a principle known as First In, First Out [FIFO]). Shelves and Bins should be arranged to prevent damage to the sterile barrier packaging, and the physical placement of items within bins and on shelves should be evaluated to reduce the risk of damage to sterile barrier packaging.

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