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A sterile medical device is one which is free of viable micro-organisms. European standards, which specify requirements for validation and routine control of a sterilization process, require (when it is necessary to supply a sterile medical device) that adventitious microbiological contamination of a medical device prior to sterilization be minimized. Even so, medical devices produced under standard manufacturing conditions in accordance with the requirements for quality management systems (BS EN ISO 13485:2003) or which have been subjected to a cleaning process as part of their reprocessing in a health care establishment may, prior to sterilization, have micro-organisms on them, albeit in low numbers. Such products are non-sterile. The purpose of sterilization is to inactivate the microbiological contaminants and thereby transform the non-sterile products into sterile ones.
The kinetics of inactivation of a pure culture of micro-organisms by physical and/or chemical agents used to sterilize medical devices can generally best be described by an exponential relationship between the numbers of micro-organisms surviving and the extent of treatment with the sterilizing agent; inevitably this means that there is always a finite probability that a micro-organism may survive regardless of the extent of treatment applied. For a given treatment, the probability of survival is determined by the number and resistance of micro-organisms and by the environment in which the organisms exist during treatment. It follows that the sterility of any one product in a population subjected to sterilization cannot be guaranteed and the sterility of a processed population is defined in terms of the probability of there being a viable micro-organism present on a product item.
BS EN 15424 describes the requirements that will enable the demonstration that a low temperature steam and formaldehyde sterilization process has appropriate microbicidal activity. Also that this activity is both reliable and reproducible, such that the relationship for the inactivation of micro-organisms can be extrapolated with reasonable confidence to low levels of probability of there being a viable micro-organism present on a product after sterilization.
Requirements of the quality management system for medical device design/development, production, installation and servicing are given in BS EN ISO 13485. The standard for quality management systems recognizes that, for certain processes used in manufacturing or reprocessing, the effectiveness cannot be fully verified by subsequent inspection and testing of the product. Sterilization is an example of such a process. For this reason, sterilization processes are validated for use, the performance of the sterilization process monitored routinely and the equipment maintained.
Exposure to a properly validated, accurately controlled sterilization process is not the only factor associated with the provision of reliable assurance that the product is sterile and, in this regard, suitable for its intended use. Attention is therefore given to a number of factors including:
- The microbiological status of incoming raw materials and/or components
- The validation and routine control of any cleaning and disinfection procedures used on the product
- The control of the environment in which the product is manufactured, assembled and packaged
- The control of equipment and processes
- The control of personnel and their hygiene
- The manner and materials in which the product is packaged
- The conditions under which the product is transported and stored.