Overview
ISO 11138-2:2017 specifies requirements and test methods for biological indicators (BIs) used to assess sterilizers and sterilization processes that employ ethylene oxide (EO) gas. The standard covers test organisms, suspensions, inoculated carriers, BI construction, and resistance test methods for EO processes operating in the temperature range of 29 °C to 65 °C.
This part of the ISO 11138 series provides the product-level requirements for BIs intended specifically for EO sterilization, complementing process validation standards such as ISO 11135 and ISO 14937 and user guidance in ISO 14161.
Key Topics
- Test organisms: Spores of Bacillus atrophaeus, Bacillus subtilis or other strains demonstrating equivalent performance are specified. Examples of suitable B. atrophaeus strains are cited in the standard.
- Population and resistance: Viable count declarations and resistance characteristics (D values) are required. For inoculated carriers and BIs the viable count shall be ≥ 1.0 × 10^6 per unit.
- D value requirements:
- D value is reported in minutes at 54 °C to one decimal place.
- For BIs with B. atrophaeus spores, D ≥ 2.5 min at 54 °C when tested using ISO 18472-compliant resistometer conditions with specified test gas mixtures.
- An alternative D ≥ 2.0 min at 54 °C applies when testing with 100% EO as the test gas.
- Resistometer method (Annex A): Describes the chamber sequence (evacuation to ~10 kPa, humidity control to ~60% ±10% RH, EO admission to ~600 mg/l ±30 mg/l, exposure timing, chamber purge) and the requirement to use reference resistometer process parameters as specified in ISO 18472.
- Carrier and packaging suitability (Clause 7.2): Exposure conditions to demonstrate suitability include a minimum exposure temperature ≥ 55 °C, sterilant concentration ≥ 800 mg/l in air at ≥ 70% RH, exposure time ≥ 6 h, and a manufacturer-declared maximum exposure temperature.
Applications
- Validation and routine monitoring of EO sterilization cycles in medical device and healthcare product manufacturing.
- Production quality control for manufacturers of biological indicators and inoculated carriers.
- Comparative assessment of BI batches by D value reporting and survival–kill window calculations (method references provided in ISO 11138-1).
Related Standards
- ISO 11138-1:2017 - General requirements for biological indicators
- ISO 11135 - Development, validation and routine control of EO sterilization processes
- ISO 14937 - General requirements for characterization of sterilizing agents
- ISO 14161 - Guidance on selection, use and interpretation of biological indicators
- ISO 18472 - Test equipment and resistometer parameters for EO testing
For users and manufacturers, ISO 11138-2:2017 provides the practical product requirements and standardized test methods necessary to ensure consistent, traceable, and reproducible assessment of ethylene oxide sterilization performance.