SOP for Ethylene Oxide (EO) Sterilization Validation
1. Purpose
To describe the procedure for validation and routine monitoring of the Ethylene Oxide (EO) sterilization process using the overkill method (AAMI/ISO 11135 Method C) to ensure that the sterilization process consistently achieves the required Sterility Assurance Level (SAL) for medical devices.
2. Scope
This SOP applies to the validation, monitoring, and revalidation of EO sterilization processes for medical devices processed through contract sterilization facilities.
3. Principle
The overkill method is based on demonstrating that the sterilization process effectively inactivates a microbial challenge greater than the product’s natural microbial contamination (bioburden). Biological indicators containing Bacillus atrophaeus spores are used as a challenge organism due to their high resistance to EO sterilization.
4. Responsibilities
Quality Assurance (QA)
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Approve validation protocol and reports
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Ensure compliance with sterilization standards
Microbiology Laboratory
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Perform bioburden testing and sterility testing
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Process biological indicators and microbial samples
Sterilization Facility
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Provide calibrated sterilization equipment
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Execute validation cycles according to defined parameters
5. Validation Requirements
Validation includes the following microbiological and process requirements:
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Validation protocol preparation
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Bioburden method validation
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Bioburden enumeration
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Bacteriostasis/Fungistasis testing
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Microbial challenge using biological indicators
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Product vs. biological indicator resistance evaluation
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External biological indicator validation
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Product load configuration
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EO cycle monitoring
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Residual EtO testing
6. Procedure
6.1 Validation Protocol
A validation protocol shall be prepared before performing the sterilization validation. The protocol must include:
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Description of the medical device
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Sterilization process parameters
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Test procedures and acceptance criteria
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Sample quantities and locations
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Biological indicator placement
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Sterility testing methods
One representative product sample shall be submitted to the testing laboratory for evaluation.
6.2 Bioburden Method Validation
The method used for routine determination of product bioburden must be validated to confirm:
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Effective recovery of microorganisms from the product
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Adequate microbial growth during testing
The validation includes two phases:
a) Adverse Substance Screening
Determine whether the product inhibits microbial recovery.
b) Recovery Testing
Performed using one of the following methods:
Repetitive (Exhaustive) Method
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Recommended: 5 non-sterile samples
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Minimum: 3 samples
Product Inoculation (Simulated) Method
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Recommended: 5 sterile samples
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Minimum: 3 samples
This study determines whether a recovery factor must be applied to routine bioburden results.
6.3 Bioburden Enumeration
Bioburden levels must be determined before sterilization.
Testing requirements:
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Aerobic bacteria and fungi testing
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10 samples from each of three production lots
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Samples should represent different production or packaging times
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Samples must be collected immediately prior to sterilization
Total recommended samples: 30 non-sterile samples
6.4 Bacteriostasis/Fungistasis Test
The sterility test method must be validated to confirm that the product does not inhibit microbial growth.
Purpose:
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Prevent false negative sterility results
Sample requirements:
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6 sterile samples
If sterile samples are unavailable, testing may be performed using samples from fractional or half cycles.
6.5 Microbial Challenge Using Biological Indicators
Biological indicators used in validation contain:
Organism:
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Bacillus atrophaeus spores
Typical BI specification:
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10⁶ CFU per spore strip
Requirements:
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Population verification for each BI lot
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Three BI replicates tested per lot
6.6 Product vs Biological Indicator Resistance
Fractional cycles are used to demonstrate that:
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Biological indicators are more resistant to EO sterilization than product bioburden organisms.
After fractional cycles:
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Both product and BI strips undergo sterility testing.
Acceptance criteria:
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Product sterility results must be negative
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BI survival may occur in fractional cycles
If confirmed, routine monitoring may rely on BI testing only in half cycles.
6.7 Biological Indicator Placement
Before EO processing:
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BIs must be placed inside the product packaging
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Locations must represent worst-case gas penetration points
BI quantity calculation (based on sterilizer chamber volume):
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20 BIs for first 5 m³
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2 BIs per m³ between 5–10 m³
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1 BI per m³ beyond 10 m³
Minimum requirement:
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20 BIs + 1 positive control BI
6.8 External Biological Indicators
External biological indicators may be validated for routine release testing.
Procedure:
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Place additional BIs outside product packages
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Process during fractional cycles
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Confirm external BI resistance is equal or greater than internal BI resistance.
6.9 Product Load Configuration
During validation cycles:
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Load the sterilizer with maximum product quantity
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Use densest load configuration
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Product containing BI samples must be distributed throughout the chamber
A load placement diagram must include:
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Product locations
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BI locations
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Temperature sensors
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Humidity sensors
6.10 EO Cycle Parameters and Monitoring
Validation cycles must be performed using sterilizers with completed:
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Installation Qualification (IQ)
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Operational Qualification (OQ)
Parameters monitored:
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Chamber temperature
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Product temperature
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EO gas concentration
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Chamber humidity
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Product humidity
Temperature sensors must include locations where BI samples are placed.
6.11 Microbial Validation – Sample Processing
Cycle procedure:
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Load product into sterilizer chamber.
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Precondition for designated minimum time.
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Run sterilization cycle.
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Remove product and BI samples after exposure.
Samples should be sent to the microbiology laboratory within the same day or next day for sterility testing.
Validation Cycle Requirements
Fractional Cycles
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1 or more cycles
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Test product and BIs (internal and external)
Half Cycles
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3 cycles
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Test BIs only
Full Cycles
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3 cycles
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Include first three routine sterilization loads
Acceptance Criteria
Validation is acceptable if:
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Product sterility tests in fractional cycles are negative
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All BIs from half cycles are negative
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Full cycle BI results confirm half cycle results
6.12 Ethylene Oxide Residual Testing
Residual EO testing must be conducted to confirm compliance with allowable limits.
Testing options:
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Final aeration timepoint analysis
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EO dissipation curve study
Recommended sampling:
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3–5 time points
Sample requirements:
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2 sterile samples per time point
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1 unsterilized control sample
This testing establishes quarantine time before product release.
7. Routine Monitoring
7.1 Routine Cycle Monitoring
Routine sterilization loads must include biological indicator monitoring.
Recommended BI quantity:
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10 BIs for first 5 m³
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1 BI per m³ between 5–10 m³
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1 BI per m³ beyond 10 m³
Minimum requirement:
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10 BIs + 1 positive control BI
Loads may be released after acceptable BI sterility results.
7.2 Routine Bioburden Monitoring
Routine bioburden monitoring ensures microbiological control of manufacturing.
Bioburden may be affected by:
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Raw materials
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Vendors
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Personnel
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Equipment
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Water systems
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Seasonal variations
Recommended frequency:
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Minimum quarterly bioburden testing
8. Revalidation
Annual Review
An annual documented review of manufacturing and sterilization processes must confirm that no significant changes have occurred.
Periodic Revalidation
The EO sterilization process must be revalidated at least every two years.
Minimum revalidation activities:
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Bioburden testing
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One sub-lethal cycle
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One half cycle
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Ethylene oxide residual testing
Full Revalidation Required If:
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Product design changes
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Packaging changes
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Manufacturing process changes
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Sterilization parameter changes