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Sterility assurance level (SAL) is a term used in microbiology to describe the probability of a single unit being non-sterile after it has been subjected to the sterilization process. SAL is also used to describe the killing efficacy of a sterilization process, where a very effective sterilization process has a very high SAL.

In microbiology, it’s impossible to prove that all organisms have been destroyed because:

  • They could be present but undetectable simply because they’re not being incubated in their preferred environment.
  • They could be present but undetectable because their existence has never been discovered.

Therefore, SALs are used to describe the probability that a given sterilization process has destroyed all of the microorganisms.

Sterilization of medical devices and ensuring that biological products are not contaminated with harmful agents requires stringent testing procedures to isolate and identify any existing contaminants.

Biological Indicator Testing

Biological indicators (BI) are filter strips impregnated with Bacillus atrophaeus spores. These strips are used to measure the effectiveness of the sterilization process. BIs are subjected to the customer’s preferred sterilization method – usually via ethylene oxide (EtO) gas, steam, or dry heat sterilization. Finally, BI strips are tested for remaining presence of the biological spores. Since such spores are challenging to sterilize, their absence in the sterilized sample validates the method or product/lot and determines sterility assurance levels.

The industry standard to be complied with for use of Biological Indicators is ANSI/AAMI/ISO 11138. ISO 11138-1:2006 provides general requirements for production, labeling, test methods and performance characteristics of biological indicators, including inoculated carriers and suspensions, and their components, to be used in the validation and routine monitoring of sterilization processes. ISO 11138-1:2006 specifies basic and common requirements that are applicable to all subsequent parts of ISO 11138. Requirements for biological indicators for specified processes are provided in the subsequent parts of ISO 11138.

Chemical Indicators

Different types of chemical indicators

Chemical Indicators are color changing strips that indicate whether the desired conditions necessary for sterilization have been achieved within the autoclave/sterilizer. This is done by placing the color changing strips within the sterilizer. When the strip changes colors, it is an indicator that the desired temperature and other conditions have been met. Chemical Indicators are popularly used for Steam, dry heat, EtO, and Radiation methods of sterilization.

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