A sterile medical device is one that is free of viable microorganisms. USP requires that “articles intended to be sterile must attain a ≤ 10-6 probability of a non-sterile unit, i.e., less than or equal to one chance in one million that viable bioburden microorganisms are present.” This is known as the sterility assurance level (SAL) or the Probability of a Non-Sterile Unit (PNSU). PNSU also describes the killing efficacy of a sterilization process. A very effective sterilization process has a very low SAL. When selecting the sterilization cycle, you need to be confident that the product will still be able to function appropriately after it undergoes the sterilization process.
There are three methods that are currently used to accomplish PNSU: overkill, bioburden/biological indicator, and bioburden-based methods. The easiest is the overkill method, but it provides the greatest impact on materials. According to USP, “overkill sterilization can be defined as a method in which the destruction of a high concentration of a resistant microorganism supports the destruction of reasonably anticipated bioburden present in routine processing.” Bioburden/biological indicator based sterilization relies on the biological indicator to show that there was incomplete destruction of the microorganisms present.
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