The half-cycle method
To support the minimum PNSU, some practitioners adopt the half-cycle approach in which a 106 population of a resistant indicator is inactivated during the validation effort, and then the exposure period is doubled
in routine operation. It relies on simple mathematics. Killing a BI with an initial population of 106 in the half-cycle validation study means at least a 9-log reduction has been attained. (A routine sterilization cycle with
double the exposure period projects to >18-log reduction). This assumes the bioburden is identical in count and resistance
to the BI, which is virtually impossible in the real world. The PNSU for a typical bioburden microorganism using this approach
could easily exceed 10–200 or greater (see Table II for an example of half-cycle using an 8-min validation cycle and 16-min routine cycle with an initial
microbial population of 106).
Figure 3 represents this process and the effect on the BI microorganism. (The D
121 value of the BI in Figures 3–6 is approximately 1 min.) The slope of the death curve, which depicts the number of surviving
microorganisms at a time point, is known precisely only during the survivor-curve region where the number of survivors can
be readily determined. The numbers of potentially surviving BI organisms in the fraction-negative region can be estimated
using various methods requiring multiple challenge units exposed at cycles where some but not all of the BIs are rendered
sterile. Extension of the death curve below a survival probability of 1 × 10–2 to 1 × 10–3 assumes that the death curve is logarithmically linear. This is a reasonable assumption because microbial death follows
essentially first-order kinetics.
The confirmation of a 1 × 10–6 PNSU using any microbial challenge available can only be assumed, but it can be assumed with a high degree of confidence
based upon biological lethality data. This salient point must be accepted without question. Irving Pflug has explained, "Accept
that while the objective of a sterilization process may be a PNSU of 10–6, we cannot directly measure microbial levels of less than one surviving microorganism in 10 to 100 units (from 10–1 to 10–2). Therefore when designing or validating sterilization processes, we use indirect methods, so we have real measurements that
are equivalent to our nonmeasurable PNSU of 10–6" (9).
In a sterilization validation study using an initial BI population of 106 spores per indicator, 20 strips per study, and three replicate runs in which no survivors are observed, the log reduction
demonstrated requires approximately 9 logs (106 × 20 × 3 = 6 × 107 reduced to zero survivors) (see Figure 4).
If this process is considered the half cycle, then the full cycle (with twice the exposure time) is depicted in Figure 5.