On the cause of performance variation of biological indicator used for sterility assurance

Variations in biological indicator (BI) lethality have been reported for several types of commercial BIs. This phenomenon has been observed among different lots of the same species and strain BIs from a single vendor. It has also been reported among BIs from different vendors but of the same species and strain that are intended to challenge the same general type of sterilization process. Although BI variability has been widely reported, the contributing factors to the variation in observed lethality have not been specifically identified. This is because the previous reports overlooked, to some extent, the differences in carrier materials, primary packaging materials, and culture media used in the manufacture of commercial BIs. The differences in lethality attributable to the carrier material, for so called "substrate effects," have been widely reported. For the BI preparation in this experiment, the same carrier material, primary packaging material, and culture medium were used. The only variable was the use of different spore suspensions supplied from different BI manufacturers. The authors found no significant difference in BI performance as measured by BI resistance. BI population may vary depending on the retrieval technique or population variability in a purchased BI suspension. Unlike some previously published studies, there was no indication from our studies that a specific BI manufacturer supplied BIs with either greater resistance or greater population.