By definition, the D-value for a biological indicator (BI) is the time (or dose) required at a specified set of exposure conditions to reduce the viable spore population by one log or 90%. Notice in the above definition that the D-value is for the BI, not the spore. One should not forget that the D-value is a measure of resistance performance that refers to the entire BI package and is not a value for the spore itself.
All of the components listed below have an impact on the resistance performance for the assembled BI. Changes to any of the components will likely result in a change in the resistance performance of the BI.
•Carrier material (paper strip, stainless steel disc, liquid suspension) on which the spores are deposited
•Primary packaging (glassine envelope, Tyvek®/mylar envelope, plastic culture tube, glass ampoule size and shape)
•Media fill volume and pH for SCBIs
•Batch and Brand of recovery media (TSB for spore strips)
•Incubation conditions for recovery of spores
In addition to the components used to manufacture the BI, slight differences in resistometer performance or programmed test parameters could lead to altered resistance performance. For more information on D-value and how it impacts BI performance, subscribe to Spore News, Mesa’s online white paper.