How To Validate Sterility In Autoclaves Vs. Radiation Sterilized Medical Products
How does the United States Food and Drug Administration (FDA) define sterile?
Under the strictest definition of sterility, an item or product is sterile when there is the complete absence of viable microorganisms (bacteria, yeasts, viruses, and molds). For regulatory purposes, sterility is defined by acceptance criteria based on calculated contamination probability. An acceptable level of contamination risk for most items is the probability of a single contaminated product out of a million manufactured products. However, sterility criteria may be more stringent or lax depending upon the intended use of the medical device or product. The most common method of sterilization is an autoclave sterilization process. Autoclave sterilization guidelines dictate an autoclave’s temperature and time. Ionizing radiation sterilization processes like gamma radiation sterilization, and electron beam radiation sterilization are great alternatives to autoclave sterilization for devices with heat-sensitive materials. This article explores both autoclave and ionizing radiation sterilization methods for medical devices and medical products.
What are sterilization validations, and why are they important?
Since the sterility of a medical device or product is based on acceptance criteria, the process that a product or device undergoes to become sterile must be validated to prove that sterility acceptance criteria are consistently met. Sterility can be assured only using a validated sterilization process under current good manufacturing practices (cGMP). Sterility cannot be demonstrated by reliance on periodic sterility testing of final products alone. Thus, sterilization validations are tests that accumulate data about a sterilization process and statistically prove that the sterilization process can consistently and effectively sterilize medical devices or products under “worst-case scenario” conditions.
What is Ionizing radiation sterilization (sterilization by radiation)?
Ionizing radiation sterilization is a non-thermal sterilization method that functions by destroying microorganisms in a product with gamma radiation, beta particles (electron beam), x-ray, or ultraviolet light. Other than sterile filtration, ionizing radiation sterilization is the only other sterilization method that doesn’t rely on elevated temperature to sterilize products. Sterilization by radiation is an excellent alternative for products that cannot be sterilized with heat or chemicals.
What items can be sterilized by radiation?
Items sterilized with radiation are the same as items that gaseous methods can sterilize. Standard devices and materials sterilized with radiation are plastics, heat-labile materials (e.g., electronics), and powders. Radiation damages the nucleoproteins of microorganisms and thus is not recommended for biologics.
What is Autoclave sterilization?
Autoclave sterilization processes are also known as steam sterilization or moist heat sterilization. Moist heat sterilization destroys microorganisms in a product with steam under pressure. Sterilization by moist heat is the most common method for medical device and medical product sterilization.
What items can Undergo Autoclave sterilization?
Items that traditionally undergo autoclave sterilization processes include rubber, durable plastic materials, mixing tanks, surgical equipment, filling equipment, freeze-dryer chambers, and filled product containers that can withstand high-temperature exposure.
What are the differences in validating Autoclave Sterilization vs. radiation sterilized products?
Autoclave sterilized products utilize an overkill method to prove an autoclave’s sterilization cycle and parameters can destroy a certain quantity of bioburden. The overkill method requires successfully killing reference microorganisms (bacterial spores) to establish a certain level of sterility. Bacterial spores are a worst-case scenario for bioburden. Thus, the lethality for sterilization cycles that pass an overkill method test will far exceed any unexpected rises in microbial contamination for manufactured products.
In contrast, products sterilized with ionizing radiation sterilization are validated through a bioburden method. The bioburden method evaluates and monitors the pre-sterilization microbial population of products to be sterilized. Through controlling and monitoring pre-sterilization bioburden, items can undergo ionizing radiation sterilization with a proven minimum dose of radiation to destroy microbes on manufactured medical devices or products.
How are radiation sterilization validations performed?
Validations for radiation sterilization use a bioburden approach versus the traditional overkill approach used with most heat-based sterilization methods. Thus, the control and monitoring of product bioburden are vital for items that will be sterilized via radiation. Interestingly, biological indicators are not used for radiation sterilization validations because product bioburden and dosimetric measurements are more reliable. The activities performed and assessed for radiation sterilization validations are detailed below.
Validations for radiation sterilization processes assess the following activities:
#1: Dose Verification
Analysts use pre-sterilization bioburden controls and regular evaluations of manufacturing process effects on bioburden levels to keep up sterilization cycle efficacy. Increases in bioburden or bacterial spore populations could increase the radiation dose necessary to provide sterility. Indeed, the bioburden’s natural radiation resistance levels and concentrations allow analysts to extrapolate a radiation dose that produces only a single nonsterile unit in a million sterilized products. During dosage verification, the radiation dosage must match the actual bioburden of the products being sterilized.
#2: Product Material Compatibility
Once the minimum dose for sterility is determined, analysts often establish the maximum radiation dose by calculating the highest likely quantity that a product could experience during the radiation sterilization process. Radiation has initial and long-term effects on materials. Thus, some materials may appear unchanged immediately following radiation exposure and then deteriorate over time. Material compatibility studies should evaluate all materials exposed to the radiation for their functionality over the product’s intended use period. For pharmaceutical products, the material compatibility of the drug product and its primary container is of greatest concern post-radiation.
#3: Equipment Qualification
Gamma sterilization systems (and other radiation sterilization systems) require the initial and regular evaluation of software and equipment controls. Examples of radiation sterilization system controls include scan speed, source intensity, and system timers. Most radiation chambers have a conveyor belt system that moves products through the radiation sterilization cycle. Thus, equipment qualifications for machines that transport products are also required.
#4: Empty Chamber Dose Mapping
Empty chamber dose mapping is not always required to validate radiation sterilization processes. Open chamber dose mapping provides a valuable performance baseline for the radiation sterilization system.
#5: Full Chamber (Load) Dose Mapping
Load dose mapping aims to determine a radiation dose distribution throughout the processed items (load items). When sterilized, products are arranged in containers, carriers, or pallets. Load dose mapping determines an item loading configuration that minimizes radiation dose variation across the product’s materials. Sterilized items are mapped using dosimeters positioned within the load undergoing sterilization (internally) and at strategic intervals within the radiation system (externally). With load dose mapping, the location of minimum dose exposure and maximum dose exposure within the group of sterilized items and within the radiation system can be identified. This dose mapping data support later routine monitoring of the sterilization process.
#6: Dosimetry Verification
Radiation sterilization relies on dosimetry for both initial development and ongoing sterilization process verification. Dosimeters give a local reading of the radiation level in a particular area of the radiation sterilization system. Dosimeters must be calibrated regularly to ISO standards to maintain their accuracy. Dosimeter calibrations should be documented both for initial sterilization validation and during routine sterilization process control and monitoring.
#6: Dosimetry Verification
Radiation sterilization relies on dosimetry for both initial development and ongoing sterilization process verification. Dosimeters give a local reading of the radiation level in a particular area of the radiation sterilization system. Dosimeters must be calibrated regularly to ISO standards to maintain their accuracy. Dosimeter calibrations should be documented both for initial sterilization validation and during routine sterilization process control and monitoring.
#7: Radiation Sterilization Process Confirmation
The primary aim of sterilization validation is to confirm the lethality of the radiation dose received by the products processed through the radiation-sterilizing equipment. Sterilization cycle efficacy is accomplished by replicating studies that prove the dosimetry results correspond to the required minimum radiation value for sterility assurance. Further, these studies must demonstrate that the maximum radiation dosage is not exceeded during the sterilization process.
How are sterilization validations for autoclave Sterilization performed?
Autoclave sterilization validations require multiple formally documented stages. The first sterilization validation stage is the process development stage. In the autoclave sterilization process development stage, operating parameters and controls used for the sterilization process are investigated and selected. The next stage is the installation qualification stage, which ensures that equipment controls and instrumentation are installed and calibrated appropriately. As part of the installation qualification, systems to regulate steam, water, and air should be verified and documented. The third sterilization validation stage is the operational qualification stage. Operational qualification makes sure that installed equipment functions within the set sterilization process parameters. After the operation of the equipment is verified, the performance qualification stage begins. Performance qualifications assess the sterilization of materials, items, and biological indicators that pass through the sterilization process under validation. Performance qualifications measure sterilization cycle controls and the effectiveness of the sterilization cycle in overcoming worst-case biological challenges. The fifth and final stage of sterilization validation is the routine process control stage. This final stage ensures that sterilization processes are continuously monitored and controlled to maintain the efficacy of product sterilization.
The overkill method is utilized as a part of the performance qualification for autoclave sterilization validations. The overkill methods are used to validate autoclave sterilization processes 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 microorganism burden that may occur during pre-sterilization 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, appropriate 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 to undergo the autoclave sterilization process in locations most challenging to achieve sterilizing conditions.
Overkill Method Partial Cycle Approach
Finding the reduced treatment point needed to inactivate one million microorganisms on an ISO 11138-3 compliant biological indicator (BI) are the autoclave sterilization guidelines for a partial cycle approach. Once found, this sterilization treatment level is performed three times to prove reproducibility. The autoclave cycle’s confirmed microorganism inactivation rate can then be used to predict the probability of microorganism survival. Probability is determined using the inactivation kinetics of the sterilizing agent and the number and resistance of the microorganisms on the BI.
Overkill Method 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 account for microbial variations and changes in microbial resistance caused by unplanned contact with contaminated material. Microorganisms with 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 a sterilizing agent for the normal cycle to confirm no survivors. Once a successful sterilization cycle is established, the overkill method is to be performed two other times to ensure the repeatability of the process. This ensures autoclave sterilization guidelines are met.
Summary
All in all, medical product and device sterility can be assured only by using a validated sterilization process under current good manufacturing practices (cGMP). Sterilization validations prove that set sterility acceptance criteria (of one unsterile product in a million or less) are consistently met. Radiation sterilization is a non-thermal method of sterilization that functions through destroying microorganisms in a product with gamma radiation, beta particles (electron beam), x-ray, or ultraviolet light. Radiation sterilization is used for medical devices and products that cannot withstand chemical or heat sterilization methods. Moist heat sterilization destroys microorganisms in a product with steam under pressure and is the most common method for medical device and medical product sterilization. Sterilization validations for radiation sterilized and steam sterilized devices differ. Radiation sterilization validations use a bioburden assessment method. In contrast, steam sterilization validations use an overkill method. No matter which sterilization method you use, ensure you choose a contract testing organization that can provide appropriate sterilization validations for your unique medical device or product needs.
Ethide Labs is a contract testing organization specializing in Sterilization Validations and Bioburden Testing. Ethide Labs also offers Microbiology Testing, Bacterial Endotoxin Testing, EO Residual Testing, Sterility Testing, Cytotoxicity Testing, Environmental Monitoring & Package Integrity 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).
Michael J. Akers. Sterile Drug Products Formulation, Packaging, Manufacture, and Quality. Drugs and the Pharmaceutical Sciences. Informa Healthcare. 2010.
United States Pharmacopeial Convention. <1115> Bioburden Control of Non-Sterile Drug Substances and Products. Rockville, MD, USA. 2021. (USPC <1115>).
United States Pharmacopeial Convention. <1116> Microbiological Control & Monitoring of Aseptic Processing Environments. Rockville, MD, USA. 2021. (USPC <1116>).
United States Pharmacopeial Convention. <1211> Sterility Assurance. Rockville, MD, USA. 2021. (USPC <1211>).
United States Pharmacopeial Convention. <1229> Sterilization of Compendial Articles. Rockville, MD, USA. 2021. (USPC <1229>).
United States Pharmacopeial Convention. <1229.10> Radiation Sterilization. Rockville, MD, USA. 2021. (USPC <1229.10>).
