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Sterilization is intended to render Reusable Medical Devices free from viable mircroorganisms
Prion are not microorganisms and are more resistant than conventional microorganisms.
Sterilization of RMD is based on 3 key concepts
Various methods are proposed by international standards to demonstrate the capability of a given sterilization process to reach the SAL using overkill method. One of them is the half cycle overkill method, described as follows:
Tests are performed with microorganisms known for their high resistance to the sterilization process (usually bacterial spores).
Once the process has been characterized, it must be verified that an RMD can be efficiently sterilized. The 106 inoculum is placed at the position determined as the most difficult to sterilize on or within an RMD. RMD's are packaged and inserted in a representative challenging load. For routine controls, inoculums may be placed in process challenge devices (PCD) | ![]() |
The sterilization process whereby an RMD is sterilized within a packaging which preserves its sterility until use is called terminal sterilization.
Terminal sterilization can be obtained at either high temperature or at low temperatures.
Steam sterilization is the most common sterilization method. It is also referred to as moist heat sterilization or saturated steam sterilization),
In dry heat sterilization, RMD are exposed to dry, hot air. Low temperature sterilization (LTS) is adapted tor RMD's, which do not withstand high temperatures. Current low temperature sterilizing agents are: Ethylene Oxide (EtO), steam formaldehyde (LTSF), vaporized hydrogen peroxide (VH2O2) and ozone (O3). RMD are exposed in controlled temperature, humidity and/or pressure conditions to a minimum concentration (Cc) of sterilizing agent during the time required for achieving the desired SAL.
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Radiation sterilization (ionizing – gamma, e-beam or high energy X-Ray, or non-ionizing ultra-violet (UV) ) is not commonly used for reprocessing of RMD’s in healthcare facilities and will not be discussed in present guidelines.
Non-terminal sterilization methods meet the SAL criteria. However, unlike terminal sterilization, RMD’s are not protected by a packaging. Immediate use steam sterilization (previously called flash sterilization) is an example of non-terminal sterilization process .
All sterilization processes require occupational health and safety precautions.
Various and size and configurations of sterilizers are offered on the market.
A sterilization method choice is made according to Spaulding classification principles and applicable local regulations or recommendations.
Common preferences or trends may be summarized as follows :
Level of flexibility is left to user for choice of the sterilization method, which depends on local regulations or guidelines. For instance, in some countries steam, is used except when not allowed by an RMD manufacturer’s IFU. In other countries, LTS is used or tolerated for steam-compatible devices known to deteriorate from repeated exposure to steam (e.g., optics).
Compliance of a sterilization process to international standards may be required by local applicable regulations.
dedicated standards are available for steam1,2,3,4,5 for dry heat6, for EtO7,8 and LTSF9,10. ISO 14937 is used for VH2O2 (dedicated standards are currently being developed)12,13. There are no directly applicable international standards for liquid sterilant processes and IUSS.
Sterilization cycles can be divided into 3 phases.
Written standard operating procedures (SOP’s) describe operation and controls to be performed before, during and after the sterilization cycle. For a newly purchased RMD, a new SOP is defined if an existing one cannot be used.
In an SCBI the biological indicator and medium required for incubation and recovery of the test microorganisms are enclosed in a sterile barrier system. After exposure, the medium is aseptically brought in contact with the BI. The answer provided by the SCBI is a binary growth no growth. Readout time ranges from 48 hours down to a few hours and even less with the new generation of BI. Fast readout BI detect enzymes which reliably simulates the behaviors of the test microorganisms. The SCBI often includes a process challenging device function. A process challenge device is defined by international standard ISO 11139 (2018) as "an item providing a defined resistance to a cleaning, disinfection, or sterilization process and used to assess performance of the process". In the case of an SCBI a tortuous path challenges the penetration of the sterilizing agent into the capsule containing the BI. To be meaningful the PCD challenge must be benchmarked against reusable medical devices intended for sterilization. BI's are placed in a challenging position in the load. |
Means must be provided by a sterilizer manufacturer to avoid a software or hardware failure of the control system, which remains undetected, and that a non-valid cycle appears valid. In practice, independent sensors double-check that process variables are within specified tolerances. For instance, in the case of steam sterilization, the temperature sensor that regulates process temperature is doubled by an independent temperature sensor. The independent sensor technology does not need to be the same as long as deviation of the control function is reliably detected. A manufacturer's IFU provides instructions to a user in case of deviances are detected. Independent data must be recorded for traceability. |
Load release (i.e. formal authorization to distribute the RDM for storage and use) is performed by an accredited person. The person in charge of load release is ideally different from the one in charge of unloading controls. Minimal requirements for load release are: Cycle parameters are within tolerances and no deviation of the control system are detected by the independent sensors Color changes of Type 1 CI are confirmed. Visual controls (damage to package, residual humidity,..etc..) are confirmed. Additional CI (other than type 1) may be required by the standard operating procedure (SOP). CI's placed in packaging are controlled at the point of use by operation room personnel. Results are not a prerequisite for load release. However, in case of a positive CI, a risk analysis is performed to determine whether some or all items must be reprocessed or recalled. When BI's are used, they are incubated and analyzed according to a BI manufacturer IFU. Healthcare facilities SOP's define measures in case of positive BI. A risk analysis determines if it is a false-positive (which is known to happen) or a true indication of an issue. Depending on the documented outcome of the risk analysis, the decision can be: a release of the load (because other control gave confidence that the cycle is conformed), a partial release or reprocessing of the entire load. Requirements for load release vary between countries and sterilization technologies. Parametric release (i.e., load release without a BI) is more common in Europe and usually linked to a more stringent observance of cycle parameters and process control. For saturated steam sterilization, it is universally accepted that, given the high margin safety, BI's are not systematically needed. Some countries like the US may recommend BI's for implants or advise daily or weekly BI controls. In most European countries, BI's are not used routinely for steam. For other sterilization technologies, activities depends on national best practices, or rules and interpretation of international standards. In any case, BI and CI are a complement to minimal requirements for load release as stated above but cannot be used to avoid control of cycles parameters, visual inspections and CI controls. |
In steam sterilization, hot saturated steam coats all RMD surfaces for a specific time set according to guidelines. Saturated means that steam is maintained at a steady state between a liquid and vapor phase with a very low percentage of the liquid phase. The Regnault table gives the pressure and temperature conditions for steam saturation.
Temperatures above 100°C, are obtained by increasing pressure in the sterilization chamber above atmospheric pressure.
A penetration test and a leak test are performed periodically as recommended by international standards, or imposed by local regulations (usually daily)
The 3 phases of steam sterilization cycles are as follows :
Dry heat achieves sterilization by means of conduction. The heat is absorbed by the RMD and moves within an RMD layer-by-layer. For the RMD to be fully sterilized, it needs to reach the required temperature. Compared to steam sterilization, dry heat T°C and time required for efficacy are higher and longer, the penetration performances are poor, and cool down-times are longer. Dry heat is banned in a growing number of countries due to its fixative properties. How to conduct dry sterilization:
Vaporized hydrogen peroxide sterilization brings RMD surfaces in contact with vaporized H2O2 (VH2O2) at a certain concentration and during the process time required for SAL. VH2O2 is obtained by vaporization of a liquid sterilant solution (usually the concentration is above 50%). Some cycles increase the VH2O2 concentration before distribution in the chamber.
Conditioning, exposure and removal schemes may be repeated once or several times. Conditioning of repetition phases may be simplified compared to the initial conditioning. Usually the total number of repeat phase (including the first one) is even. The cycle ends with a final removal usually reinforced compared to previous removals.
Formaldehyde (HCHO) is a colorless gas, highly soluble in water. Formaldehyde is obtained by vaporization of a solution at various concentrations of formaldehyde (below 35%). Inactivation power of formaldehyde is greatly improved by the presence of humidity.
EtO is a colorless, poisonous gas that attacks the cellular proteins and nucleic acids of microorganisms. EtO process temperatures range from 25 – 55°C. A lower temperature results in a less efficient process and a longer exposure time. EtO is carcinogenic to humans and flammable. Special room conditions, safety equipment and separate ventilation systems are required.
Immediate use steam sterilization (IUSS) is a variant of steam sterilization intended for emergency point-of-use non-terminal sterilization. RMD’s are not packaged. Transfer to user is immediate and performed with care in controlled environment. Compared to steam sterilization, RMD are not packaged, drying and cooling are usually shortened to limit cycle time. So, an RMD may hence still be wet on cycle completion, thereby increasing the risk of environmental contamination.
The 3 phases of IUSS are as follows :
Some countries do not allow IUSS, others may tolerate it. Recommendations usually advise to make an evaluation regarding the need for point-of-use reprocessing. RMD inventory may have to be improved to afford reprocessing by a central sterilization department.
There are no international standards for IUSS.
Local regulations or guidelines may require or recommend that sterilization takes place in centralized sterilization department thereby prohibiting point of use sterilization. Operating theatres are usually not equipped and organized to clean and dry RMD’s as consistently as a centralized sterilization department.
Exposure of all RMD surfaces to a liquid sterilant for a controlled time, temperature and concentration yields a targeted SAL. Rinsing must preserve the SAL.
The most common sterilant is peracetic acid. Cleaning is usually separated from sterilant application.
Acceptance of the liquid sterilant concept depends on the region. In some countries, it may be accepted or tolerated as an alternative to high or low temperature sterilization for some fragile or heat sensitive RMD’s. In other countries, this practice may be considered as a deviation to Spaulding classification principles.
Written cleaning and disinfection standard operating procedures (SOP’s) are prepared according to quality management principles.
User supervises or performs and controls a process validation :
Per terminal sterilization methods, steam at 134°C or 132°C is the preferred sterilization temperatures and may be used when specified by an RMD manufacturer’s IFU. Exposure time vary according to country regulations or guidelines.
Dry heat sterilization should be replaced by steam.
For RMD’s that are not compatible with steam at 132°C or 134°C , an RMD manufacturer’s IFU indicates the sterilization methods between either steam at 121°C to 125°C or low temperature sterilization and cycles to be used. Choice of LTS methods is according to regulatory, guidelines, practical or occupational health and safety considerations.
Liquid sterilization claims remain to be evaluated.
IUSS to be replaced by steam sterilization
Go to IUSS sterilization →
1 of 16 Liquid sterilantTo be evaluated by WFHSS
Go to Liquid sterilization →
2 of 16 SteamSteam 134°C or 132°C preferred when allowed by RMD IFU
Go to Steam sterilization →
3 of 16 LTSFCycle according to RMD IFU
Go to Low temperature steam formaldehyde →
4 of 16 VH2O2Cycle according to RMD IFU
Go to Vaporized H2O2 →
5 of 16 EtOCycle according to RMD IFU
Go to Ethylene Oxide →
6 of 16 Clean, dry, packaged RMDNon packaged for non terminal sterilization
Go to choice of sterilization process →
7 of 16 Sterile Medical DeviceNon packaged RMD for immediate use when non terminal sterilization
Packaged RMD for storage when terminal sterilization
Go to choice of sterilization process →
Terminal sterilization preferred to
non terminal
Go to choice of sterilization process →
9 of 16 Non terminal sterilizationThe RMD is not protected by a packaging and must be immediately used after sterilization
Go to choice of sterilization process →
10 of 16 +Terminal sterilization preferred
Go to recommendation of WFHSS for sterilization →
11 of 16 +Steam sterilization at 132°C or 134°C preferred when allowed by RMD IFU
Go to recommendation of WFHSS for sterilization →
12 of 16 +Visual control and routine controls
Go to Sterilization and quality management →
13 of 16 +According to RMD IFU
Go to choice of sterilization process →
14 of 16 +Steam sterilization at 132°C or 134°C preferred when allowed by RMD IFU
Go to recommendation of WFHSS for sterilization →
15 of 16 +Visual control and routine controls
Go to Sterilization and quality management →
16 of 16Copyright 2021 Ⓒ WFHSS / Development by Benedetto.S
“The WFHSS executive committee is pleased to put the WFHSS Guidelines at your disposal.
They have been written for all the professionals working directly or indirectly in the field of the reprocessing of Reusable Medical Devices (RMD) used in Health Care facilities.
They are the result of a consensus from a review of national practices, standards, regulations.
They are intended to provide guidance and the state of the art recommendations from an academic world society focused on science but their purpose is not to supersede local regulation, standards or guidelines.
They will be updated regularly to follow the evolution of the science and the evolution of the RMD and the technologies.
Your feedback is essential to contribute to the improvement of the Guidelines , feel free to use the form to send your comments or suggestions.
We wish you interesting reading!
On behalf of the Executive Committee
Dr Christine DENIS, President”
“The WFHSS executive committee is pleased to put the WFHSS Guidelines at your disposal.
They have been written for all the professionals working directly or indirectly in the field of the reprocessing of Reusable Medical Devices (RMD) used in Health Care facilities.
They are the result of a consensus from a review of national practices, standards, regulations.
They are intended to provide guidance and the state of the art recommendations from an academic world society focused on science but their purpose is not to supersede local regulation, standards or guidelines.
They will be updated regularly to follow the evolution of the science and the evolution of the RMD and the technologies.
Your feedback is essential to contribute to the improvement of the Guidelines , feel free to use the form to send your comments or suggestions.
We wish you interesting reading!
On behalf of the Executive Committee
Dr Christine DENIS, President”
For the present guidlelines reusable medical device (RMD) means:
The reprocessing of single use medical device is outside the scope of theses guidelines.
Medical device regulations vary between regions. Some items may not be registered as medical devices in some regions.
Sterilization is intended to renders the reusable medical device free from viable microorganisms. Sterilization is implemented on a clean RMD. Most common Sterilization process is steam. Low temperature sterilization processes are available for heat sensitive RMD
Disinfection :
Process to reduce the number of viable microorganisms to a level previously specified as being appropriate for a defined purpose (ISO 11139 : 2018)
Cleaning :
Removal of contaminants to the extent necessary for its further processing or for intended use (ISO 11139 : 2018)
Other definition: The first step required to physically remove contamination by foreign material, e.g. dust soil. It will also remove organic material such as blood, secretion, excretion and microorganisms, to prepare a medical device for sterilization or disinfection (WHO : 2016)
Cleaning may be combined to disinfection in cleaning & disinfections processes (for instance in automated washer-disinfectors)
Sterilization :
Process used to render product free from viable microorganisms (ISO 11139 : 2018)
Reusable Medical device (RMD)
Medical devices wich are not single use i.e. which can be reused under appropriate reprocessing conditions, for an undelimited number of time or for a predetermined number of use.