Understanding the sterility assurance level
To understand how steam sterilization processes are validated, it is important to understand some key concepts about sterilization. The purpose of sterilization is to inactivate microbiological contaminants, thereby rendering products sterile. The kinetics of inactivation of microorganisms by sterilizing agents are best described as an exponential relationship between the number of microorganisms surviving and the extent of treatment with the sterilizing agent1. Consequently, there is always a probability that a microorganism may survive regardless of the extent of treatment applied and sterility of any one product in a population subjected to sterilization processing cannot be guaranteed1. Instead, the sterility of a processed population is defined in terms of the probability of there being a single viable microorganism present on a product item1. This is referred to as the sterility assurance level (SAL). The SAL is expressed as a quantitative value which can be interpreted as the probability of a device being non-sterile after exposure to a sterilization process. For example, an SAL of ≤ 10-6 means that there is a probability of one in a million devices being non-sterile after exposure to a sterilization process.
Validating SAL using biological indicators
Steam sterilization processes use saturated steam, under pressure for a specified exposure time and at a specified temperature, as the sterilizing agent. FDA states that for reusable devices intended to be sterilized, at least one validated microbicidal method for sterilization must be specified. For reusable items, where the challenge to the sterilization process is difficult to define and pre-sterilization treatments such as cleaning are difficult to control and are variable, the sterilization method applied should be conservative and often referred to as “overkill”, and an SAL of ≤ 10-6 should be obtained1, . To prove an SAL of ≤ 10-6, a moist heat sterilization cycle can be validated using a partial-cycle or full-cycle approach. When validating the sterility assurance level using either of these methods, the drying time is not programmed. This is done to limit the amount of lethality delivered to the BI after the exposure phase is complete. Consequently, drying times are validated separately.
The partial-cycle or ‘half-cycle’ approach for testing the sterility assurance level requires the complete inactivation of an appropriate test organism with a microbiological challenge or Fbio of ≥ 6 minutes at half-cycle parameters. Triplicate testing should be carried out to demonstrate reproducibility1. Geobacillus stearothermophilus spores are the most commonly used for the validation of moist heat sterilization processes2,. The microbiological challenge or Fbio is calculated as D121 x log10 (N0). The D121 value or decimal reduction value is an expression of resistance and is the time required to achieve inactivation of 90% (or 1 log) of a population of the test microorganism at 121°C. N0 refers to the pre-exposure viable population of the biological indicator. The biological indicators or inoculated carriers used must comply with ISO11138-3:2017, which means that they must have a viable count of ≥ 1,0 x 105 and a D121 value ≥ 1.5 minutes. Additionally, the z value should be ≥ 6°C. Finally, placement of the biological indicators or inoculated carriers must occur on the “most difficult to sterilize” locations within the load.
As an alternative to using the partial-cycle approach, the full-cycle approach can be used. Here, a full sterilization cycle should result in the complete inactivation of biological indicators with a microbiological challenge or Fbio of ≥ 12 minutes plus the addition of a safety factor. This safety factor is achieved by determining the log of the population required to obtain a microbiological challenge of 12 minutes and adding 0.5 log101. Like what is true for the partial-cycle approach, triplicate testing to demonstrate reproducibility is required. Biological indicators or inoculated carriers, often G. stearothermophilus, should be placed on the most difficult to sterilize locations within the load and must comply with the criteria outlined in ISO 11138-33.
Drying time validation
In addition to establishing the sterilization process through microbiological/ temperature profiling data, a minimum drying time for terminal sterilization methods should be validated; given that moisture remaining on the packaged products after sterilization could compromise package integrity and performance1,3. Devices and packaging should be visibly dry following steam sterilization4. Additionally, after steam sterilization any weight gain percentage is evaluated. There are several weight gain acceptance criteria, however, ISO/TS 17665-2:2009 Annex A and EN 285 indicate that the moisture remaining in the load at the end of a sterilization process may not result in a weight gain of more than 1% for textiles, 0.2% for metal. ISO/TS 17665-2:2009 Annex B and ANSI/AAMI/ST8:2013/(R)2018 state that the remaining moisture may not result in a weight gain of 3% for absorbable materials.
ISO/TS 17665-2:2009 states that whenever biological indicators are used to confirm lethality, the physical parameters measured during the sterilization process should always be used to verify that the defined sterilization process has been carried out4. ISO/TS 17665-2:2009 Annex A also specifies acceptance criteria for temperature profiling, stating that the accepted sterilization temperature range has the defined sterilization temperature as a lower limit and an upper limit of + 3°C. Additionally, the equilibration time should not exceed 30 seconds. For example, a cycle defined at 132°C for 4 minutes must achieve a temperature range of 132°C to 135°C for the last 3 minutes and 30 seconds of the exposure phase. Whereas ISO/TS 17665-2:2009 Annex A is suggested as informative in the US, it is often seen as a requirement in the EU. In our experience, FDA will place less emphasis on temperature data and more emphasis on microbiological data, although rejections due to the absence of temperature data have occurred. We currently perform temperature profiling separately from the other validations to ensure that there is no interference of the probes with the other tests.
 ISO 17665-1:2006 Sterilization of healthcare products – Moist heat – Part 1: requirements for the development, validation and routine control of a sterilization process for medical devices.
 AAMI TIR12:2020 Designing, testing, and labeling medical devices intended for processing by health care facilities: A guide for device manufacturers.
 FDA 2015 Reprocessing Medical Devices in Health Care Settings: Validation methods and Labeling – Guidance for Industry and Food and Drug Administration staff.
 ISO/TS 17665-2: 2009 Sterilization of health care products – Moist heat – Part 2: Guidance on the application of ISI 17665-1
 ISO 11138-3:2017 Sterilization of healthcare products – Biological indicators – Part 3: Biological indicators for moist heat sterilization processes.
 The z value is the change in temperature required to generate a 1-log change in the D value.
 ISO 14161:2009 Sterilization of health care products – Biological indicators – Guidance for the selection, use and interpretation of results
 BS EN 285: 2015 Sterilization – Steam sterilizers – Large sterilizers
 ANSI/AAMI/ST8:2013(R)2018 Hospital steam sterilizers