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BS EN ISO 11138 — Biological Indicators for Sterilization of Health Care Products
Sterilization of health care products — ISO 11138 series for biological indicators
🦠 BS EN ISO 11138 is the core standard series for the validation of health-care-product sterilization. It specifies the design, preparation, performance requirements and test methods for biological indicators (BIs) — standardised microbial preparations of stable, reproducible resistance, inoculated on a defined carrier, used to verify the killing capability of a sterilization process during installation, operational and performance qualification, and to monitor routine sterilization cycles. BSI adopted the series as the identical EN ISO 11138 text, with UK participation by Technical Committee CH/198 (Sterilization and associated equipment and processes); the parts were harmonised in a 2017 update.
💡 Biological indicators are the “gold standard” of sterilization validation: chemical indicators only indicate that a sterilant parameter was reached, whereas a BI directly demonstrates that microorganisms have been killed. BI results are the decisive criterion in release and validation.
1. Series Overview and Scope 📌
This series applies to biological indicators used in the sterilization of health care products. Part 1 gives general requirements: definitions, classification, composition (carrier and spores), labelling, storage, test methods (D-value, survival curve, kill time, etc.) and quality records; subsequent parts specify the standard microorganisms (indicator strains) and their resistance requirements for different processes: ethylene oxide, moist heat, dry heat, and low-temperature steam with formaldehyde (LTSF).
The series is used together with the process-specific standards (ISO 11135 for EO, EN ISO 17665 for moist heat, EN ISO 20857 for dry heat, etc.) to support acceptance and release.
⚠️ The BI resistance (D-value) must match the parameters of the sterilization process being validated. Using a mismatched BI (too resistant or too susceptible) distorts the conclusion — too resistant over-specifies the process, too susceptible masks a process deficiency.
2. Series Structure and Part List 📑
| Part | Year | Subject | Status |
|---|---|---|---|
| Part 1 | 2017 | General requirements (definitions, test methods) | Readable text |
| Part 2 | 2017 | BIs for ethylene oxide sterilization processes | Readable text |
| Part 3 | 2017 | BIs for moist heat (steam) sterilization processes | Readable text |
| Part 4 | 2017 | BIs for dry heat sterilization processes | Scan — written from public domain knowledge |
| Part 5 | 2017 | BIs for low-temperature steam and formaldehyde (LTSF) processes | Scan — written from public domain knowledge |
3. General Requirements and Resistance Determination 🔧
Part 1 specifies the common requirements for biological indicators. Key terms include: inoculum (number of viable spores on the carrier), D-value (time required to reduce the microbial population by one log unit under specified conditions), survival curve and kill time (time for complete kill). Requirements include: carrier and packaging must not impede sterilant penetration; the spore suspension must have a stable concentration and resistance; the label must state strain, lot number, quantity, D-value and storage conditions; the manufacturer must provide a certificate of analysis and maintain process-capability data.
🚨 D-value determination must be carried out under strictly controlled conditions (temperature, concentration, humidity and method all as specified) and its uncertainty reported. Large discrepancies between laboratories or methods indicate a flawed measurement process requiring investigation.
4. Process-Specific Biological Indicator Requirements 🛠️
- Ethylene oxide (Part 2): specifies indicator strains such as Bacillus atrophaeus and their resistance range, considering the combined effect of EO concentration, temperature and humidity.
- Moist heat (Part 3): specifies moist-heat-resistant strains such as Geobacillus stearothermophilus for typical 121°C/134°C steam cycles.
- Dry heat (Part 4): specifies strains applicable to dry heat (e.g. depyrogenation ovens above 160–180°C) and the test methods.
- Low-temperature steam formaldehyde (Part 5): specifies the indicator strains and resistance for LTSF low-temperature equipment validation.
💡 Process validation usually uses the indicator microorganism with the highest resistance to the sterilant as the worst-case representative (e.g. G. stearothermophilus for moist heat). If the worst case is inactivated, other microorganisms are inferred to be inactivated too.
5. Engineering Practice and Application Notes 📏
In sterilization validation and routine monitoring, BIs are applied as follows: define the target process and indicator microorganism → place BIs at the hardest-to-sterilize (challenge) locations in the load → run the process, then incubate and inspect for survival → judge process effectiveness against the acceptance criteria. For routine batch release, self-contained BIs (with culture medium and colour indicator) can give rapid results.
- Challenge placement: place BIs where the sterilant is hardest to reach (e.g. load centre, long lumens) and record the placement diagram.
- Incubation and controls: run negative and positive controls with each batch to exclude false negatives/positives of the culture system.
- Traceability: retain BI lot numbers, D-value certificates, placement diagrams, incubation records and release decisions for regulatory audit.
✅ Correct BI use substantially improves the confidence in the Sterility Assurance Level (SAL): in validation reports, annual requalification and deviation investigations, this series is the standard citation basis.
6. Frequently Asked Questions ❓
❓ Q: What is the difference between a biological and a chemical indicator?
A: A chemical indicator changes colour to signal that a sterilant parameter (temperature, time, steam, etc.) has been reached; a biological indicator directly demonstrates that the process killed a defined number of standard microorganisms. BIs are closer to the actual purpose of sterilization but require incubation for results.
A: A chemical indicator changes colour to signal that a sterilant parameter (temperature, time, steam, etc.) has been reached; a biological indicator directly demonstrates that the process killed a defined number of standard microorganisms. BIs are closer to the actual purpose of sterilization but require incubation for results.
❓ Q: What is a D-value and why does it matter?
A: The D-value is the time required to reduce the microbial population by one log unit (90%) under specified conditions, reflecting the resistance of the microorganism. It is the key parameter for setting the cycle, calculating the SAL and selecting the indicator strain.
A: The D-value is the time required to reduce the microbial population by one log unit (90%) under specified conditions, reflecting the resistance of the microorganism. It is the key parameter for setting the cycle, calculating the SAL and selecting the indicator strain.
❓ Q: Can a BI be reused?
A: No. A biological indicator is a single-use verification item; after use it is treated as biohazardous waste. Any “reuse” destroys its calibrated resistance and sterility, invalidating the result.
A: No. A biological indicator is a single-use verification item; after use it is treated as biohazardous waste. Any “reuse” destroys its calibrated resistance and sterility, invalidating the result.
❓ Q: When must biological indicators be used?
A: Process validation (IQ/OQ/PQ), requalification, revalidation after process change and deviation investigations generally require BIs; routine release may place BIs in batches depending on risk assessment and regulatory requirements.
A: Process validation (IQ/OQ/PQ), requalification, revalidation after process change and deviation investigations generally require BIs; routine release may place BIs in batches depending on risk assessment and regulatory requirements.