There are many factors that can affect the efficacy of a sterilization cycle. These factors can include the concentration of the sterilizing agent, temperature, relative humidity, pH, and duration of exposure time. Since there are so many variables, it was necessary to come up with an industry-wide method in regards to a correlation between standardized set of conditions during a set exposure time. The answer to this comes in the form of D-value.
D-value refers to decimal reduction time. This is the time it takes in a specified set of conditions to kill 90% of the exposed microorganisms in the biological indicator. USP chapter <55> goes into great detail about the procedure for performing D-value tests – from the number of replicates required (20), the number of exposure conditions (at least five), and the difference in sterilizing times over a series (NMT 75% of the expected D-value).
This chapter also discusses the recovery process. Within 4 hours of exposure, you are required to remove the BIs and add each of them to a suitable medium. Each tube needs to be incubated for 7 days, at an optimal recovery temperature for the BI type used.
To calculate the D-value, you may use one of three methods: Limited Spearman-Karber, Survival Curve Method, or Stumbo-Murphy-Cochran.
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