Biological indicators

The theory behind the BI is that if your process is effective enough to kill a large population of highly resistant spores, it will also kill a lower number of less resistant organisms on the medical devices. Because it detects the killing of microbial spores, a BI is capable of yielding information that is more valuable than any other sterilisation monitor.

After exposure to the sterilisation process, self-contained BIs are activated to provide the spores with optimised growth media, and then must be incubated at the appropriate temperature to determine if any spores survived. For this reason, results are not immediately available. However, advances in BI technologies have led to rapid readout BIs, which provide the end user with actionable results in less than 30 minutes.

Sterile is the state of being free of all living microorganisms and is usually expressed as a probability.¹ A sterility assurance level (SAL) is a number that tells you the probability of a viable microorganism being present on an item after sterilisation.¹ This number expresses the likelihood that a microorganism survived the sterilisation. An SAL of 10-6 (or 10 to the negative sixth power) indicates there is one chance in a million that a device is not sterile and is generally accepted as appropriate for an item intended to contact compromised tissue.¹

The steriliser manufacturer is responsible for providing a steriliser and process that can achieve the desired SAL and uses BIs as a tool to estimate SAL and develop and validate their sterilisation processes. The user is responsible for monitoring the performance of the steriliser (e.g., with a BI) to ensure it is operating as intended¹.

References:

1. CDC Guideline for Disinfection and Sterilisation in Healthcare Facilities, 2008; https://www.cdc.gov/infectioncontrol/pdf/guidelines/disinfection-guidelines-H.pdf [accessed June 27, 2023].
2. ANSI/AAMI ST79:2017 Comprehensive guide to steam sterilisation and sterility assurance in health care facilities.