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.


RRS company designs and validates all BIs according to ISO 11138:2017 specifications. This standard specifies the quality parameters that should govern the BI performance. In addition to the population size, ISO 11138:2017 defines the resistance of the indicator through the D-Value. The D-value for a BI is the time (or dose) at a specified set of exposure conditions that will result in a 1 Log (or 90%) reduction to the viable spore population. The D-value is a measure of spore resistance to a particular sterilization process: the higher the D-value, the more resistant the microorganism is to destruction. ISO 11138:2017 Annexes C and D suggest different methods for the D-Value calculation.

Content source:

ISO 11138:2017 Sterilization of health care products — Biological indicators

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Steam Biological Indicator 10^5 (RRS 14-21110)

Steam Biological Indicator 10^6 (RRS 14-21120)