Models of sterility assurance.
There are several models that can be applied to achieve sterility assurance. In the ISO approach, (mainly applied in hospitals
and in the manufacture or treatment of medical devices) conventional worst-case devices are defined. For example, stacks of
tissue of defined dimensions or hollow tubes of defined diameter and length are loaded with BIs or chemical indicators of
saturated steam. These devices are placed at arbitrary positions in ill-defined sterilizer loads. When BIs are inactivated
after a sterilization cycle, the cycle is considered effective.
The approach taken under GMP regulations is different. It is expected that each product-specific sterilization cycle is validated
separately. The sterilizer load must be defined and the worst-case position must be characterized for each process. Sterilization
effectiveness of the cycle should be correlated to the effect obtained at the true worst-case position and not to the effect
obtained in a conventional worst-case device.
A third option seems to be favored by some regulators at present. The product to be sterilized is expected to be manufactured
under extremely stringent conditions to ensure an extremely low presterilization bioburden. This is apparently seen as important
because of the lack of confidence in the validation of the sterilizing effect obtained under worst-case conditions. The goal
of this approach is to minimize the probability of survival at ill-defined worst-case conditions by minimizing the presterilization
Definition of worst-case positions and worst-case conditions.
The effectiveness of steam sterilization is influenced by a number of critical factors. Sterilization temperature and exposure
time are the only factors that are considered in F-value or F0-value calculations of sterilization processes. It must be clear that such calculations are valid only when all other factors
that influence the inactivation of microorganisms are duly considered. Steam quality is a critical factor in all cases in
which steam comes in direct contact with the product or surface to be sterilized. Steam quality may be of minor significance
where steam is used only as a means of heat transfer and where heat exchange is achieved rapidly by conduction or radiation.
The worst-case position in a sterilizer load is where the sum of all the influences on microorganisms, including the effect
of the product or the influences of the microenvironment results in minimal inactivation. The conditions achieved at that
worst-case position are the worst-case conditions for the sterilization process.
Worst-case positions can be determined only in studies using bacterial endospores during product and process development because
the worst-case positions are difficult to predict. The worst-case conditions should be simulated in BI studies as closely
as possible and the sterilizer conditions needed to achieve the required effect therein should be reflected in the parameters
to be measured when the sterilization process is monitored.
The term overkill cycles for highly effective sterilization cycles is misleading and should be abandoned. Following the USP definition, the Ph.Eur. standard cycle for steam sterilization is an overkill cycle. It is sufficient to inactivate 15-log scales of a resistant
microorganism with a D-value of 1 min. If the D-value of the BI is 1.5 min (as defined in Ph.Eur.), then the inactivation is only 10 logs, which means that it is just sufficient to deliver the kill time for a BI with 106 viable spores/unit. If the area between the stopper and the glass wall of a vial is taken as the worst-case position, then
the cycle might not even kill 6 logs of endospores of the most resistant environmental isolate, and the cycle may qualify
for a bioburden-oriented cycle at best.
A sterilization cycle in of itself cannot be considered an overkill cycle unless the effect is related to a given situation
in which a given maximum number of organisms of a given maximum resistance under defined worst-case conditions is considered.
A risk-based approach.
Worst-case positions of loads or equipment to be sterilized and the worst-case conditions achieved therein must be specified
for each sterilization cycle. Because these are the conditions in which the least biological effect is achieved, quantitative
studies on inoculated bacterial endospores are needed to investigate and determine the minimal lethal effect achieved by a