Because the lethality of the full cycle cannot be demonstrated using a biochallenge and recalling that a 106 BI challenge is destroyed in the half cycle, the log reduction delivered to the BI must be assumed. Using the assumed log
reduction for the half cycle and doubling the dwell time, the log reduction for the BI in the full cycle can be estimated
at 18 logs and easily meets the overkill definition of a minimum 12-log reduction of a BI with a D-value of 1 min (assuming the BI had a D
121 > 1 min) as is typical in nearly all overkill validation studies.
There are several inherent assumptions with the half-cycle method. The first assumption is that the bioburden would be as
numerous and resistant as the BI. Second, it is assumed that the complete destruction of the multiple BIs in triplicate studies
demonstrated a 9-log reduction of that BI. Finally, the death curve of the BI is assumed to be linear in a region where it
cannot be experimentally determined.
Sterilization process objective
The routine sterilization of items in any sterilization process is intended to destroy the bioburden microorganisms that might
be present on or in the materials being processed, regardless of their initial population and resistance. The goal is constant:
to attain a minimum PNSU of 10–6. Consider the following definition of overkill sterilization as being consistent with what process expectations should be:
Overkill sterilization is a process where the destruction of a high concentration of a resistant microorganism supports the
elimination of bioburden that might be present in routine processing. That objective can be demonstrated by attaining any
of the following: a defined minimum F
0, a defined time-temperature condition, or a defined log reduction of a biological indicator.
This definition reflects the process requirement directly, with full recognition that bioburden organisms typically have minimal
heat resistance. Destruction of the BI in high concentration requires time and temperature conditions far in excess of what
is required to destroy the bioburden, and thus overkill is demonstrated.
Demonstrating a minimum PNSU of 1 × 10–6 for a sterilization process can be ensured only where the number and resistance of the microorganisms present on or in the
items being sterilized is known. This can be accomplished definitively using any of the sterilization-cycle approaches described
previously, and delivering that lethality is not restricted to the overkill method. It is supported by information about the
relative resistance of the bioburden to the biological indicator in the BB/BI method in which partial kill of the indicator
is sufficient to support the required minimum PNSU for the bioburden. With the overkill method, complete destruction of the
resistant BI in high numbers is more than sufficient to ensure the minimum PNSU for any conceivable bioburden.
In everyday usage of sterilizing equipment, the BI is not present. The process is expected to confidently destroy the bioburden.
In today's industry, we have numerous controls on the pre-sterilization bioburden, and in many instances, especially terminal
sterilization, it is monitored for each sterilizer load.