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EtO Sterilization: Microbiological Aspects of Process Validation

Deliberate decision making during the structuring of microbial challenges, product loads, and biological indicators can provide a validation process for EtO sterilization that ensures accuracy, the absence of microbes, and a smooth testing procedure.

Susan Edel Satter and Paul J. Sordellini

A companion to this article, EtO Sterilization: Principles of Process Design, discussed the components of each phase of two 100% EtO with nitrogen processes, focusing on the engineering aspects of designing EtO cycles.¹ This article focuses on various approaches to medical device sterilization cycle validation from a microbiological standpoint. The discussion assumes that the first two stages of the validation—the commissioning and the physical performance qualification of the sterilization chamber—have been successfully completed. Therefore, this discussion deals solely with the third stage of EtO validation: the microbiological performance qualification (MPQ). The topics that are discussed include how to choose the appropriate microbial challenge for an EtO sterilization process, approach to developing the EtO cycle, product load, placement of biological indicators in/on the product load, method for determining cycle lethality, and calculations to determine the D-value.

Information is also given on the documentation for the report on validation and certification of the process, and revalidation is discussed briefly. Some suggestions exceed the current requirements presented in international standards, but they can enhance a validation process, resulting in a more thorough and accurate study.

International standards ISO 9001 and 9002 present the quality system requirements for the design, development, production, installation, and servicing of healthcare products. The ISO 9000 series treats medical device sterilization as a special manufacturing process because the results cannot be verified by inspecting and testing 100% of the product at the conclusion of the cycle. Sterilization processes must, therefore, be assessed for special considerations, validated prior to use (or during use in certain situations), and routinely monitored.

To design an effective validation and routine control program for a sterilization process, the bioburden on the product and packaging must be considered. Bioburden is defined as "the population of viable microorganisms on a product and/or a package" and is characterized in terms of number, identity, and resistance. A validated test method must demonstrate that it can consistently and adequately remove the bioburden from the product and packaging. There are various bioburden test methods and associated validation procedures from which to choose.²

Many factors can contribute to the bioburden on product and packaging, including the origin of the raw materials and components, transit and storage conditions, and the manufacturing environment in which the finished products are assembled and packaged. European standards place particular emphasis on controlling the processes used to manufacture sterile products.³

Sterility is defined as the "state of being free from viable microorganisms."⁴ Microbial death relating to the gaseous sterilization of healthcare products is an exponential function typically defined as the probability of a nonsterile item existing per a given number of units in a batch. This probability, called the sterility assurance level (SAL), defines the "probability of a viable microorganism being present on a product unit after sterilization."⁴ While sterilization can reduce the bioburden on a given product to a very low number, that probability can never be reduced to zero. Therefore, in order to achieve the desired bioburden levels, it is critical to design a validation program that provides a high degree of confidence for consistent sterilization.

Click here to proceed to page 2, Determining the Appropriate Microbial Challenge

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