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Understanding the Reduced Incubation Time (RIT) report

Biological indicators (BI) are considered the highest standard for monitoring the sterilization process. BI use spores (the most resistant life form) to verify the sterilization cycle lethality. During the last decade, the sterilization market has faced numerous challenges; such as the need for faster reusable materials’ reprocessing, the development of new medical devices and instruments, the emergence of new technologies for their reprocessing, and the need for new indicators appropriate for their control.

In the face of these new challenges, BI manufacturers have invested their efforts in reducing biological indicators incubation time and thus accelerating the release of the processed material. Today, it is possible to find BIs with a readout time between 72 hours to 15 minutes.

ISO11138-1 states that the incubation time must be validated by the manufacturer of the biological indicator. A method for validating a reduced incubation time (RIT) was published in Annex II of “Biological Indicator (BI) Premarket Notification [510(k)] Submissions” Guidance from FDA. This protocol is appropriated for all types of BIs with a readout time less than 7 days.

Sometimes, customers misinterpret this protocol and assume that they must perform the RIT in their own sterilizer. The RIT study is the responsibility of the manufacturer of the BIs and not the end users. The protocol sets out how to validate the RIT for a BI, but it is the manufacturer who decides how often to perform it.

The guidance requires BI manufacturers to expose a total of 300 BIs (100 BIs from 3 different batches) to partial conditions in the appropriate sterilization cycle in order to produce a mixture of positive and negative results. The manufacturer has to identify a partial sterilization cycle in which 30%-80% of biological indicators survive. A partial cycle is described as a cycle in which all sterilization parameters, except time, are met.

The rapid readout was compared to the 7‐day incubation result to determine the reliability of the rapid result. Result reliability was determined using the statistical term of sensitivity. Sensitivity is a measure of the accuracy of the rapid readout to indicate spore survival after sterilization process failure, in comparison with the indicator response after 7-day incubation.


A false negative is defined as a biological indicator that shows a negative response after the rapid incubation and a positive response after a 7-day incubation time.

In summary, and according to FDA guidance, the manufacturer has to demonstrate that at least 97% of the 7‐day growth visual positives were detected by the rapid readout. In other words, the manufacturer has to demonstrate that the system sensitivity is greater than or equal to 97%.

Terragene has demonstrated a sensitivity greater than 97% in the results of all its indicators with readout time different to 7 days. As an example, the results of the SCBI BT224 (final readout in 20 minutes at 60° C) are shown below:

Bionova® BT224 Self-Contained Biological Indicator
Steam Sterilization Processes
Validation of Reduced Incubation Time – Readout Reliability Summary

Each indicator with a readout different from 7 days has a RIT validation study, which the user can request to Terragene and thus have proof of the product accuracy.


  1. ISO 11138-1:2006: Sterilization of health care products. Biological Indicators. General requirements.
  2. ISO 18472:2006: Sterilization of health care products. Biological and Chemical Indicators. Essay equipment.
  3. FDA guidance document “Biological Indicator (BI) Premarket Notification [510(k)] Submissions”, attachment II.