How to validate steam sterilization processes for medical devices?
What is considered sterile for sterilization validation?
Under the strictest definition of sterility, an item or product is sterile when there is a 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 contamination for one in a million products. However, sterility criteria may be more stringent or lax depending upon the intended use of the medical device or product.
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 sterilize medical devices or products under “worst-case scenario” conditions.
What is steam sterilization (moist heat sterilization)?
Sterilization by moist heat (also known as steam sterilization) destroys microorganisms in a product with pressurized steam. Sterilization by moist heat is the most common method for medical device and medical product sterilization. However, autoclaves cannot sterilize medical devices with heat and humidity-sensitive materials.
What items can be steam sterilized in an autoclave?
Items traditionally sterilized by moist heat include durable plastic materials, glass items, metal items (mixing tanks), surgical equipment, filling equipment, ceramic items, elastomeric materials (such as rubber), freeze-dryer chambers, and filled product containers that can withstand thermal degradation at high temperatures. Often, medical devices are wrapped in cloth when sterilized to maintain their sterility outside of the autoclave until use.
How do you validate a steam sterilization process?
The overkill method is the number one approach for steam sterilization validation. Overkill sterilization validates sterilization processes by killing high concentrations of resistant microorganisms. The resistant microorganism concentration is higher than the anticipated product bioburden obtained during routine medical device or product manufacturing. Acceptance criteria include meeting a defined minimum lethality (F0), a set of physical conditions, or confirmation of a minimum log reduction of a biological indicator. A minimum log reduction of biological indicators is the most common acceptance criteria for an overkill method.
The validation requirements for the overkill method are simpler and easier to execute than other validation methods such as bioburden or a combined bioburden-biological indicator approach. Sterilization valuations for steam require several steps, including equipment qualification, empty chamber performance, component mapping, load mapping, biological indicator selection, heat penetration with microbiological challenge, and routine process control maintenance. All these seven steps are described in further detail below.
Step 1: Equipment qualification
Equipment qualifications examine autoclave equipment to confirm proper installation and operation. A sterilizer’s preventive maintenance and change control functionality standard is based on the initial equipment qualification. Thus, equipment qualifications are essential for the operational excellence of sterilization equipment like autoclaves.
Step 2: Empty chamber performance
Empty chamber performance assessments are part of a sterilizer’s installation qualification and maintenance. For this assessment, each air removal method for the sterilizer model is evaluated for temperature control and distribution. Temperature measurements near the corners of the sterilizer chamber and any other critical areas are collected. Thus, this assessment can determine a distribution of temperatures inside the empty chamber. Only the shortest dwell period for each air removal method is evaluated for temperature distributions. The sterilizer’s capabilities and desired sterilization cycle use determine the acceptance criteria for this test.
Step 3: Component mapping
Medical devices and other items sterilized with steam have complex structures and surfaces that can be tricky to sterilize appropriately. Examples of device or component structures resistant to sterilization are interior void volumes, obscured surfaces, and small channels or crevasses. Due to the different designs and topographies of medical devices and products, each component to be sterilized must be mapped for its resistance to steam penetration. Component mapping is most difficult for filling assemblies, filter housings, tubing, and hoses. In component mapping studies, studies are completed to identify item cold spots. If cold areas are identified, the sterilization cycle must be adjusted to ensure appropriate heat penetration throughout all sterilization load components. Heat penetration is assessed using thermocouples in contact with the item’s surface. Component mapping must be performed for each autoclave device that items are sterilized within. For component mapping studies, medical devices and other products must be similarly wrapped and oriented so that there is limited variation in steam ingress and condensate removal between cycles.
Step 4: Load mapping
Each item loading pattern requires different sterilization cycle requirements in order for all items to be sterilized. The denser the item packing, the less space there is between surfaces. With closer spacing between objects, steam struggles to penetrate all product areas and sterilize the products. Of greatest concern is the loading pattern for the terminal sterilization of aqueous liquids. Aqueous liquids must be placed in specific areas of the autoclave for accurate sterilization results. Steam sterilization of other items, like many medical devices and surgical tools, is more forgiving. Maximum and minimum sterilization load criteria can be established using item mass or number. Generally, it is recommended to load oversized items on the lower shelves, which supports condensate removal during the sterilization cycle. If large items are placed higher in the autoclave, condensate may spill onto items below before being removed via drain or vacuum.
Step 5: Biological indicator selection
Most biological indicators for steam sterilization contain spores of Geobacillus stearothermophilus (ATCC 12980 or ATCC 7953). Geobacillus stearothermophilus is a heat-loving bacterial spore form with high resistance to death via steam sterilization. Biological indicators can be located on a substrate within or on an item to be sterilized. Alternatively, a medical device or product can be challenged with a high concentration of a spore suspension and then loaded into the sterilization chamber. If using purchased biological indicators, microbial resistance information provided by the vendor can be used for the sterilization validation. Otherwise, technicians must calculate the spore population and microbial spore resistance.
Step 6: Heat penetration with a microbial challenge
Step 6 is where the overkill method most comes into play. This validation activity aims to confirm that heat penetration for the sterilization cycle can adequately kill biological indicator microbes. The exposure time, temperature, or both are often reduced slightly from routine set points to prove sterilization cycle lethality under less optimal conditions. During overkill testing, biological indicators are placed with loaded items at the most challenging locations to sterilize. These locations may be historic or come from component mapping data. Thermocouples are placed in contact with the item’s surface in areas with low heat penetration. When thermocouples and biological indicators are placed, they must not prevent steam from interacting with the objects being sterilized. Microbial challenge assessments are repeated until statistical proof of cycle overkill efficacy is established (e.g., when the biological indicators are consistently killed with the sterilization cycle).
Step 7: Routine process control maintenance
After validation, steam sterilization processes must be kept up to maintain their accuracy and efficacy over time. Process control steps include regular personnel training, equipment calibration, sterilization cycle data measurements, and preventative maintenance. As the equipment and process age, method control and change control must be kept current.
Summary
Steam sterilization is the cheapest and the most ubiquitous sterilization method. Steam sterilization kills microbes through exposure to pressurized steam in sterilization chambers known as autoclaves. Since the sterility of a medical device or product is based on acceptance criteria, autoclave cycle sterility can be assured only by using a validated sterilization process (overkill method) under current good manufacturing practices (cGMP). An acceptable level of contamination risk for most items is a probability of contamination for one in a million products.
Steam heat validations have seven essential steps: equipment qualification, empty chamber performance, component mapping, load mapping, biological indicator selection, heat penetration with microbiological challenge, and routine process control maintenance. All in all, if outsourcing your sterility testing, ensure you choose a contract testing organization that can support you with sterilization validations for your unique medical device or product needs.
Ethide Labs is a contract testing organization specializing in Sterilization Validations and Sterility Testing. Ethide Labs also offers Bioburden Testing, Bacterial Endotoxin Testing, EO Residual Testing, Microbiology Testing, Cytotoxicity Testing, Environmental Monitoring & Package Integrity Testing services for medical device companies and allied industries. Ethide is an ISO 13485 certified facility.
References
United States Pharmacopeial Convention. <1229> Sterilization of Compendial Articles. Rockville, MD, USA. 2021. (USPC <1229>).
United States Pharmacopeial Convention. <1229.1> Steam Sterilization By Direct Contact. Rockville, MD, USA. 2021. (USPC <1229.1>).
