Biological indicators (BIs), as defined by ANSI/AAMI and ISO, are test systems containing viable microorganisms providing a defined resistance to a specific sterilization process. A biological indicator provides information on whether necessary conditions were met to kill a specified number of microorganisms for a given sterilization process, providing a level of confidence in the process. Endospores, or bacterial spores, are the microorganisms primarily used in BIs. They are considered some of the toughest ones to kill. Additionally, bacterial spores are chosen for a specific sterilization process based on their known resistance to that process. For example, Geobacillus stearothermophilus spores demonstrate a high resistance towards steam and vaporized hydrogen peroxide and are therefore used in BIs that monitor these sterilization processes.
Another quality parameter used is the Z-Value. This parameter is associated with thermal sterilization processes, and its formal definition is the change in the exposure temperature that changes 10 times the D-value. The Z-value is an indicator of the biological indicator sensitivity to thermal treatments. To calculate the Z-Value, the D value is evaluated at three different temperatures. This D-value vs temperature data in log-linear regression analysis and from a graphical perspective, the Z-value is the negative reciprocal of the slope of the line of best fit.
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