Overview
ISO 11138-4:2017 - "Sterilization of health care products - Biological indicators - Part 4: Biological indicators for dry heat sterilization processes" specifies requirements for biological indicators, inoculated carriers, test organisms and test methods used to assess dry heat sterilization performance at temperatures between 120 °C and 180 °C. The standard defines how to produce, test and report resistance characteristics (D and z values) for biological indicators intended for dry heat cycles.
Key topics and requirements
- Scope & organisms
- Test organisms: spores of Bacillus atrophaeus or strains of demonstrated equivalent performance.
- Population and viability
- Inoculated carriers/indicators must have a viable count ≥ 1.0 × 10^6 per unit.
- Viable counts reported with increments ≤ 0.1 × 10^n per unit.
- Resistance metrics
- D value (decimal reduction time) is expressed in minutes at 160 °C; each batch/lot reported to one decimal place.
- Minimum D value requirement for B. atrophaeus spores: ≥ 2.0 min at 160 °C.
- z value (temperature change required for tenfold change in D) must be determined over at least three temperatures between 150 °C and 180 °C and be ≥ 20 °C.
- Test methods
- Use of a resistometer per ISO 18472 and ISO 11138-1 procedures for D and survival-kill determinations.
- Annex A: method for dry heat resistance testing (load samples, preheat chamber, controlled exposure, rapid cool, transfer to growth medium).
- Annex B: regression method to calculate z value and coefficient of determination (r^2).
- Carrier & packaging challenge
- Packaging suitability must be demonstrated: exposure conditions include minimum temperature ≥ 5 °C above manufacturer’s stated max, dry heat in ambient air, exposure time ≥ 30 min, maximum temperature as stated or ≥ 180 °C.
Applications and users
- Intended for:
- Manufacturers of biological indicators and inoculated carriers.
- Sterilization validation laboratories and QA/QC teams.
- Medical device manufacturers using dry heat sterilization.
- Regulatory bodies and conformity assessment personnel.
- Practical uses:
- Validation and routine monitoring of dry heat sterilizers.
- Comparing batches of biological indicators using stated D and z values.
- Informing selection and interpretation of biological indicator results (see ISO 14161 for guidance).
Related standards
- ISO 11138-1:2017 - General requirements for biological indicators.
- ISO 18472 - Test equipment (resistometer) for biological and chemical indicators.
- ISO 20857 - Validation and control of dry heat sterilization processes.
- ISO 14161 - Guidance on selection, use and interpretation of biological indicators.
Keywords: ISO 11138-4:2017, dry heat sterilization, biological indicators, Bacillus atrophaeus, D value, z value, resistometer, sterilization validation.