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ISO 25424:2018/Amd 1:2022 – Low Temperature Steam and Formaldehyde Sterilization of Medical Devices
Requirements for development, validation and routine control of LTSF sterilization processes
Principles of Low Temperature Steam and Formaldehyde Sterilization
ISO 25424 specifies requirements for the development, validation, and routine control of low temperature steam and formaldehyde (LTSF) sterilization processes for medical devices. LTSF sterilization is a critical technology for processing heat-sensitive medical devices that cannot withstand conventional steam sterilization temperatures. The process operates at temperatures typically between 55°C and 80°C, using a mixture of steam and formaldehyde gas as the sterilizing agent.
LTSF sterilization fills a vital niche in healthcare processing. Unlike ethylene oxide (EtO) sterilization which requires long aeration times, LTSF offers shorter cycle times while maintaining compatibility with a wide range of medical materials including plastics, rubbers, and electronics.
The sterilization mechanism relies on the alkylation reaction of formaldehyde with microbial proteins and nucleic acids. Formaldehyde (CH2O) in the gaseous phase penetrates packaging materials and device surfaces, irreversibly cross-linking proteins and DNA/RNA structures, leading to microbial death. The presence of steam enhances the biocidal effectiveness by providing the necessary humidity for optimal formaldehyde activity.
| Process Parameter | Typical Range | Criticality | Monitoring Method |
|---|---|---|---|
| Temperature | 55°C – 80°C | High | Calibrated thermocouples |
| Formaldehyde concentration | 2 – 10 mg/L | High | Chemical analysis / indicator strips |
| Relative humidity | 70% – 100% | Medium | Humidity sensors |
| Sterilization hold time | 30 – 120 min | High | Process timer + records |
| Aeration time | 8 – 24 hours | Medium | Residual formaldehyde monitoring |
Validation and Routine Control Requirements
The standard mandates a comprehensive validation framework that includes installation qualification (IQ), operational qualification (OQ), and performance qualification (PQ). During IQ, the sterilizer equipment is verified to meet manufacturer specifications and installation requirements. OQ involves testing the equipment across its operating range to establish process parameters. PQ demonstrates that the process consistently produces sterile devices under specified conditions.
Validation must consider the worst-case challenges for the sterilization process, including maximum load configuration, minimum cycle parameters, and the most difficult-to-sterilize device types. Failure to identify and test worst-case conditions is a common deficiency in LTSF validation studies.
Biological indicators (BIs) play a central role in LTSF validation. The standard requires the use of bacterial spores with established resistance to formaldehyde, typically Geobacillus stearothermophilus or Bacillus subtilis. These BIs are placed at defined locations throughout the load, including the most challenging penetration points, and are processed through a full sterilization cycle. Complete inactivation of the biological indicator demonstrates process efficacy.
The 2022 amendment (Amd 1) introduced several important clarifications to the standard. Notably, the definition of “medical device” was updated to include “in or on the human body” language, aligning with broader regulatory definitions. The amendment also corrected the definition of “inactivation curve” to match ISO 11139:2018 terminology and fixed cross-references throughout the document.
The amendment’s clarification of product release criteria was particularly important. Chemical indicators can be used for product release if complete color change is documented, and biological indicators provide the definitive evidence of sterilization efficacy. These changes improve alignment with current regulatory expectations.
Process Monitoring and Quality Assurance
Routine control of LTSF sterilization requires monitoring of critical process parameters for each cycle. Temperature, formaldehyde concentration, humidity, and exposure time must be recorded and compared against validated parameters. Any deviation from specified ranges requires investigation and potential reprocessing of the load.
The standard also addresses environmental and safety considerations. Formaldehyde is classified as a carcinogen, and the standard includes requirements for workplace exposure monitoring, emission controls, and residual formaldehyde limits on processed devices. Table D.1 in the standard provides environmental aspect analysis across the product lifecycle.
Formaldehyde residue limits are critical for patient and operator safety. Processed devices must undergo adequate aeration to reduce residual formaldehyde to safe levels. The aeration phase should be validated as part of the overall process qualification to ensure that devices are safe for patient use.
For routine release of sterilized loads, the standard specifies a combination of physical parameter review, chemical indicator results, and biological indicator testing when used as part of the release protocol. The frequency of biological indicator testing in routine production depends on the criticality of the devices being processed.
Q1: What types of medical devices are suitable for LTSF sterilization?
LTSF is ideal for heat-sensitive devices such as endoscopes, plastic instruments, electrical equipment, and rubber components that would be damaged by conventional steam sterilization at 121°C or 134°C.
LTSF is ideal for heat-sensitive devices such as endoscopes, plastic instruments, electrical equipment, and rubber components that would be damaged by conventional steam sterilization at 121°C or 134°C.
Q2: How does LTSF compare to ethylene oxide (EtO) sterilization?
LTSF offers shorter cycle times (several hours vs. days for EtO) and a less toxic sterilizing agent. However, LTSF has lower penetration capability and is not suitable for all device geometries. Both methods require careful control of process parameters and residual limits.
LTSF offers shorter cycle times (several hours vs. days for EtO) and a less toxic sterilizing agent. However, LTSF has lower penetration capability and is not suitable for all device geometries. Both methods require careful control of process parameters and residual limits.
Q3: What are the key changes in the 2022 amendment?
The amendment updated the medical device definition to include “in or on the human body,” corrected the inactivation curve definition to align with ISO 11139:2018, fixed cross-references in environmental tables, and clarified product release criteria for chemical and biological indicators.
The amendment updated the medical device definition to include “in or on the human body,” corrected the inactivation curve definition to align with ISO 11139:2018, fixed cross-references in environmental tables, and clarified product release criteria for chemical and biological indicators.
Q4: How often should biological indicators be used in routine LTSF processing?
The frequency depends on the device criticality and regulatory requirements. Typically, biological indicators are included at least weekly or with each load for implantable devices. Some regulatory frameworks require BIs in every load for critical devices.
The frequency depends on the device criticality and regulatory requirements. Typically, biological indicators are included at least weekly or with each load for implantable devices. Some regulatory frameworks require BIs in every load for critical devices.
