Why Biological Indicators Survive A Validated Cycle

By Garrett Krushefski, Laboratory Manager

When biological indicators (BIs) are exposed in a validated sterilization cycle, it is expected that all test units will be sterile. A review of the cycle performance shows that all critical parameters were achieved. Regardless, the BIs are activated and incubated as usual and for the next several days of incubation the exposed BI results are recorded. Hopefully you will never see a positive BI (yellow color) except in the case of positive controls. But where do you turn if one day you remove the samples from the incubator and staring back at you is an unexpected positive (yellow) test unit?

Part of being a manufacturer of high quality biological indicators (BI) is providing excellent support and service to the users of these products. Distributing a quality product and subsequent failure to answer questions regarding its proper use and interpretation of results falls short of what it means to be a quality manufacturer.

Recently I heard from a client who expressed the following concern about EZTest® Steam, self-contained biological indicators. The sterilization cycle in question was 7-minutes at 134°C. The exposed indicators tested negative (purple) at 48-hours of incubation. A few of the exposed units would, however, test positive (yellow) at subsequent post 48-hour observation. Finally, the client mentioned that in the past they used EZTest Steam with population of 10⁵ per unit without experiencing “failure.” They have since switched to EZTest 10⁶ and occasionally see the post 48-hour positive unit appear.