A Risk-Based Approach to the Use of Biological Indicators in the Development and Control of Steam-Sterilization Processes

Validating the sterilization process is extremely important in pharmaceutical manufacturing. The authors explore different types of sterilization processes and discuss the importance of finding the worst-case positions of loads or equipment to be sterilized and the worst-case conditions for each sterilization cycle. Biological indicators (BIs) can be used to simulate worst-case scenarios and determine the effectiveness of a particular sterilization process.
May 01, 2007

In discussions between colleagues dealing with steam-sterilization processes in the pharmaceutical industry, the medical device industry, or in hospitals, it frequently becomes obvious that sterility assurance and the use of biological indicators (BIs) as tools for the validation of sterilization cycles is not a commonly well understood and clear concept. Although it may not be surprising that sterilization is regarded differently in hospitals than in the canning industry, the differences in the healthcare sectors are more difficult to understand. Validation of sterilization processes is regarded quite differently in hospitals and in the manufacture of medical devices than in the manufacture of pharmaceutical products. It is even more confusing that within the pharmaceutical industry, the view on validation of sterilization processes and the use of BIs is not the same on both sides of the Atlantic. This article elucidates some reasons for the surprising variations in understanding the verification of sterilization effectivity by BIs.

It is not entirely clear to everybody why BIs are used at all. BIs intended for monitoring and controlling sterilization processes are preparations of bacterial endospores that are highly resistant to a particular sterilization process. They are used to demonstrate the sterilizing effect of the process. As such, BIs contain endospores that are much more resistant and present in a far larger number than the microorganisms encountered in the presterilization bioburden of any product to be sterilized. For that reason, bioindicator studies often are considered irrelevant, especially for so-called overkill processes. Is this correct, and, if so, why are we using such irrelevant sterilization procedures?

Another issue concerns the significance of the BI results obtained when monitoring or validating a sterilization cycle. Is the killing of BIs the ultimate proof of cycle validity? Or is a sterilization cycle invalid when a BI has survived this sterilization cycle? If the validation of sterilization cycles is truly simple, why are we going through a huge effort to develop and validate sterilization cycles?

One question frequently asked in the European pharmaceutical industry is: Why is it not sufficient to use qualified equipment and utilities and run a cycle that is effective enough to kill every microorganism present? When this overkill effect is verified by the routine addition of a few BIs, why should there be a need to validate specific cycles? This approach is typically taken in hospitals and in the medical device industry. The logical reverse argument also is frequently raised: Do we need biological indicators at all, or isn't it sufficient to simply use physical measurements of temperature, pressure, time, and steam quality to characterize a steam-sterilization process?

Sterilization processes, like all other processes, can be validated only when their possible problems and pitfalls are well understood. BIs and other methods or tools can be correctly used only with a clear understanding of what is intended by their use and with the identification of the strengths and limitations of the tool. There are many different steam-sterilization processes that require different validation strategies, and understanding the use of BIs is much more complicated than it may initially appear.

Sterilization processes

Sterilization processes using saturated and nonsaturated steam. In the generally accepted scientific opinion, the full effect of steam sterilization is achieved only by saturated steam in a process where heat is effectively transferred by condensation at the surface of the autoclaved products or on the surface of sterilized equipment in combination with the hydrating effect of the condensate. Although this is a correct description of the general physical phenomena that occurs in steam-sterilization processes, it is not always what happens in an autoclave. It also is an oversimplification of the real process in many cases.

In cases in which product is autoclaved in the final sealed containers, condensation of saturated steam may be a very effective method of transferring energy to the surface of the containers, but this is not the primary sterilization process. The relevant sterilizing conditions for the product itself will be generated inside the sealed containers. As an extreme example, dry-heat conditions always will be achieved in empty fused ampules regardless of how they are heated. For the same reason, it does not make sense to use self-contained spore preparations in sealed glass ampules to evaluate a process that relies on steam saturation. The degree of steam saturation is irrelevant for the sterilizing effect in this case. The device will react to heat input no matter how the heat is supplied. There can be no differentiation among dry heat, heating in an oil bath, or saturated steam. Any thermoelement would do the same job, be easier to handle, and give immediate and more-accurate results.