Abstract

Microbiological Performance Qualification – The Use of Biological Indicators

According EN 556, 1.4.1, the theoretical probability of 1 surviving microorganism in a total of 10⁶ sterilized items is required for a sterile medical device (= sterility assurance level [likelyhood of 1 non sterile in a total of 1 000 000 sterilized items]), SAL ≤ 10^-6 [1]). Sterilisation methods are described, e. g. steam sterilisation, ≥ 15’/121 ^°C, or irradiation sterilisation with a dose ≥ 25kGy where microbiological performance qualification is recommended but not required [2]. In reality, microbiological performance qualification are carried out even if it is not required, e.g. steam sterilisation [3]. In these cases, the “theoretical probability” (see above) will – inter alia – be proven by means of biological indicators (bioindicators [BI]): BIs are exposed to the sterilising agent, here steam. After a certain period of time has elapsed, a proportion of BIs is removed from the sterilising agent and the number of surviving spores are determined in relation to the initial number of spores/BIs. The spore inactivation should follow a straight line, 1^st order inactivation kinetic, when the number of surviving spores is plotted semilogarithmically against the sterilization time, preconditioned that the spore population on the BI is distributed homogeneously.

>This quantitative method has its experimental limitation which is at approximately 5 surviving spores/BI [4]. This limitation does not allow the experimental proof of the required SAL of ≤ 10^-6. Thus, from the last plot of the inactivation curve it needs to extrapolate to a SAL of 10^-6 assumed that the inactivation still follows a 1^st order kinetic. A BI to be used in steam sterilisation is normally “spiked” with 5 x 10⁵ spores of G. stearothermophilus. From these 5 x 10⁵ to the required 10^-6 only approx. 5 decimal steps can be proven experimentally, 5 x 10⁵ to ~ 5, whereas from 5 to 10^-6 one has to extrapolate.
This habit preconditions that the 1^st order kinetic will always be valid. There are data available which might prove that this assumption is not correct in all cases. Thus, microbiological performance qualification studies might have to be reconsidered.

Korrespondenz:

Dr. Michael Pfeiffer, Boehringer Ingelheim Pharma GmbH&Co. KG, Div. Human Resources Germany (HPZ 4329-EG-03), Binger Straße 173, 55216 Ingelheim (Germany), e-mail: Michael.Pfeiffer@boehringer-ingelheim.com