Understanding BI COA is one of the critical task for Non-Microbiology professional. I will be come up with number of sequel articles to understand the complete BI COA. In this article we majorly discussed about the survival time and killing time BI’s.
ISO 11135-1 states that complete kill must be obtained in the half-cycle. Those who are not familiar with BIs may think that complete kill is obtained within 6 SLRs. The logic being that the “overkill method” is centered around achieving 12 SLRs, which is equivalent to a Sterility Assurance Level (SAL) of 10^-6; therefore, half of 12 is 6. So, it is automatically assumed that in 6 SLRs (the half-cycle) all spores are killed, but this is NOT true.
6 does not equal 8, Complete kill occurs at 8 SLRs:
Theoretically, for a BI with a 1 x 10^6 spore population (at 6 SLRs of the population), there would be one surviving spore per BI. However, there is not going to be exactly one spore per BI—some are going to contain one while others may contain zero, two or three spores. Statistically, this works out to 63% of the BIs being positive.
- At the log of the population + 1 (or 7 SLRs), statistically 10% of the exposed BIs would be positive.
- At the log of the population + 2 (or 8 SLRs), statistically 1% of the exposed BIs would be positive.
If the target is to have complete kill at the half-cycle, you should expose the units until the time point at which 8 SLR is achieved. Exposing the BIs to this time period (8 SLR) is approximate because you do not know at which time point prior to the cycle time the BIs were actually killed. For this reason, we suggest you run a cycle in which there is fractional kill and then slowly increase the cycle until complete kill is achieved. However, total kill at 8 SLR is sufficient if the legwork in developing a cycle has been completed for which the internal or embedded BI is killed in the half-cycle.
Spore log reduction (SLR)
Total kill of all BI’s will appear approximately within an 8-log reduction, which is approximately at the theoretical kill time.
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