Overkill Method For Sterilization Validation
The overkill method describes the process of overkilling reference microorganisms in order to prove a certain level of sterility. This method is used to validate sterilization and to sterilize re-usable products. Overkill supports a sterilization process designed to exceed the treatment required to achieve a certain level of sterility, thus accounting for variances in pre-sterilization microorganism burden during cleaning procedures. Two types of overkill methods can be performed. One involves a full cycle approach and the other involves a reduced level of treatment known as a partial cycle approach. An example of a partial cycle approach is a half cycle approach. In order to perform an overkill sterilization cycle, biological indicators (or live microorganisms) are placed in product areas that are most difficult to sterilize and are likely to pick up a high level of bioburden (such as device lumens). Next, products are packaged routinely and loaded for sterilization in the location most difficult to achieve sterilizing conditions. Before performing a sterilization validation, make sure these contributing factors to successful sterilization have been addressed. For alternatives to the overkill approach, please see our article on the top sterilization validation methods. This article contains information on what is the overkill method, overkill sterilization, and sterilization validation protocols.
Partial Cycle Approach
For a partial cycle sterilization validation protocol, the extent of reduced treatment needed to inactivate 106 microorganisms on an ISO 11138-3 compliant biological indicator (BI) is confirmed. This treatment level is performed three times in order to prove reproducibility. An extrapolation can be used with the confirmed 106 inactivation to predict the probability of microorganism survival using the inactivation kinetics of the sterilizing agent and the number and resistance of the microorganisms on the BI.
Full Cycle Approach
For a full cycle approach, the sterilization load should be exposed to the sterilizing agent under normal conditions designed to deliver a particular level of sterilization.
The population on the biological indicators used should exceed by at least 0.5x log to the base of 10 of the population, calculated from Fbio 12 and certified D121 value for the biological indicator. This accounts for microbial variations and changes in D value caused by contact with contaminating material. Microorganisms with a high resistance to moist heat that are suitable for use include. G. sterarothermophilus, B. coagulans, C. sporogenes and B atrophaeus.
Sterilization load is then exposed to sterilizing agent for the normal cycle to confirm that there are no survivors. Once a successful sterilization cycle is established, the overkill method is to be performed 2 other times to confirm repeatability of the process.
Fbio calculations
Fbio = D121 (log No – log N)
Where:
- D121 is the D value of a BI at an exposure temp of 121°C
- No is the pre-exposure viable population of the BI.
- N is the post-exposure viable population of the BI.
Overkill Summary
Overall, both the full and partial overkill method are effective ways to validate a sterilization process for a particular medical product. The partial overkill method proves sterilization through reducing the amount of time the product is under sterilization. The full overkill method proves sterilization through checking the capacity of a traditional cycle to kill large volumes of steam resistant microorganisms.
Ethide Laboratories offers sterilization validation testing via a half cycle overkill method.
Ethide Labs is a contract testing organization that specializes in Sterilization Validations and EO Residual Testing. Ethide Labs provides in-vitro cytotoxicity tests in-house and outsources in-vivo cytotoxicity work for toxicity testing of medical devices, products, and drugs. Ethide Labs also offers Microbiology Testing, Bioburden Testing, Environmental Monitoring, Bacterial Endotoxin Testing, Package Integrity Testing & Cytotoxicity Testing services for medical device companies and allied industries. Ethide is an ISO 13485 certified facility.
References
International Organization for Standardization. Sterilization of health care products- Moist heat- Part 1: Requirements for the development, validation, and routine control of a sterilization process for medical devices. Geneva (Switzerland): ISO; 2006. (ISO 17665-1:2006/(R)2016).
