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Sterility Assurance Level (SAL) - A quick intro
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In the case of Ethylene Oxide sterilization the method for validating the sterilization process can follow ISO 13485 or EN 550. These standards require that a number of Biological Indicators (BI) be placed within the product in an orientation that is deemed “worst case scenario”. Many manufacturers will validate using product that has been seeded with the microorganism that is used in the Biological Indicators. For Ethylene Oxide sterilization the bacterium used is bacillus subtilis.
Bacillus subtilis is a microorganism that is know to be very resistant to ethylene oxide sterilization. Why use it? Because you hope that the organism will be more resistant than the worst types of microorganisms that live on you product. Hence this is one of the reason why you would want to know the types of microorganisms on your product.
The amount of bacillus subtilis on each BI will be in the range of 106 but never less than one million. The actual number of organisms on each lot of Biological Indicators should be validated by sending them to laboratory. The lab will enumerate them and send you a certificate. There are a number of reasons you should not count on the certificate that is issued by the BI manufacturer alone. One is that of control. Biological Indicators should be controlled and stored in temperature/humidity environment that is recommended by the manufacturer. This environment should be monitored to ensure compliance. Out-of-specification changes in environment should result in a new enumeration by an accredited laboratory.
HALF CYCLE
During sterilization validation - for Ethylene Oxide sterilization, the product is introduced into a chamber and then exposed to half the anticipated sterilization cycle. The biological indicators are retrieved and then evaluated for complete sterility. If you will recall the sterilization process occurs logarithmically. If all the biological indicators result in no growth it is assumed that the process had subjected the product to a sterility effectiveness to reduce the roughly 1 million bacteria to 100 or 1. You would be expected to see one BI fail if you start off with a million BI strips (which is never the case).