ISO 11737-2

ISO 11737-2, published by the International Organisation for Standardisation (ISO): “Sterilisation of healthcare products – Microbiological methods – Part 2: Sterility testing for the definition, validation and maintenance of a sterilisation process”, i.e. “Sterilisation of healthcare products – Microbiological methods – Part 2: Sterility testing for the definition, validation and maintenance of a sterilisation process”. The current version is ISO 11737-2:2019 (third edition).

Note on regulatory status: This standard defines the suitability of methods and the conduct of sterility tests carried out as part of the validation and maintenance of the sterilisation process. It is not intended for routine product release, but rather for the validation of the sterilisation process and the maintenance of its validity. For routine release, where applicable, sterility tests in accordance with the pharmacopoeias (USP <71>, EP 2.6.1, JP 4.06) are used. The standard is applied in conjunction with the sterilisation standards (ISO 11135, ISO 11137, ISO 17665, ISO 14937). The current version and the status of national adoption should be checked before submission to the competent authorities.

Classification within the ISO 11737 series

The ISO 11737 series, comprising three parts, covers methods for the microbiological evaluation of medical devices:

ISO 11737-1: Determination of microbial load (bioburden)
ISO 11737-2: Sterility testing (as part of sterilisation validation)
ISO 11737-3: Testing for bacterial endotoxins

Together with sterilisation validation (ISO 11135, ISO 11137, ISO 17665, ISO 14937) and biological evaluation (ISO 10993 series), form a comprehensive system for ensuring sterility and safety.

Basic principle

The sterility test is based on the detection of living microorganisms in samples taken from sterilised products. The basic principle of the test is as follows:

The sample is transferred to a suitable liquid culture medium (or washing solution), ensuring that the internal and external surfaces of the product come into contact with the culture medium.
The culture medium is incubated under aerobic and/or anaerobic conditions at a suitable temperature (usually 30–35 °C or 20–25 °C) for a sufficient period of time (usually 14 days).
At the end of the incubation period, cloudiness (visible growth) is observed; the presence of growth is interpreted as a failure of sterility.
A subculture is prepared from the samples in which turbidity is observed, in order to identify the proliferating microorganism.

Methods

ISO 11737-2 defines two basic methods:

Direktübertragungsmethode (Direct Transfer): Das gesamte Produkt oder ein repräsentativer Teil davon wird direkt in das Nährmedium überführt. Diese Methode eignet sich für feste, unregelmäßig geformte oder nicht in das Nährmedium eintauchbare Produkte.
Membrane filtration process: The product is washed or rinsed with a suitable liquid; the resulting liquid is filtered through a membrane with a pore size of 0.45 µm and transferred into the culture medium. Suitable for liquid products, hollow-body samples and washable devices.

Method Suitability and Validation

The validity of the sterility test depends on demonstrating the suitability of the method. Key elements:

Bacteriostasis/fungistasis test: Demonstration that the product does not inhibit the growth of a small number (usually 10–100 CFU) of microorganisms on the selected culture medium
Test microorganisms: Typically strains such as Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, Clostridium sporogenes, Candida albicans and Aspergillus brasiliensis
Wash-out/transfer yield: evidence of how much of the microorganisms was recovered from the product
Suitable incubation conditions: aerobic/anaerobic conditions, temperature and duration
Negative control: Monitoring the sterility of the culture medium and contamination of the test environment
Positive control: confirmation of the viability and sensitivity of the test system

Following validation, the sterility test shall be carried out in a repeatable manner under the specified conditions throughout the definition, validation and maintenance of the sterilisation process.

The difference between ISO 11737-2 and the sterility tests set out in the Pharmacopoeia

Whilst ISO 11737-2 focuses on sterility testing carried out as part of sterilisation validation, the sterility tests specified in the pharmacopoeias (USP <71>, EP 2.6.1, JP 4.06) define the routine release testing of pharmaceutical products. For medical devices, provided the product is supplied in a sterile state, the appropriate approach is selected depending on regulatory requirements and the type of product. The methodological principles of ISO 11737-2 are largely consistent with the methods set out in the pharmacopoeias.

The key parameters reported in the study are as follows:

Product description, sterilisation procedures and validation context
Sterility test method used (direct transfer or membrane filtration)
Culture medium(s) used and incubation conditions (temperature, duration, atmosphere)
Number of samples and sampling plan
Data on method suitability/validation (bacteriostasis/fungistasis, recovery)
Test microorganisms used
Negative and positive controls and their results
Visual assessment and detection of bacterial growth following incubation
Identification of results in the case of bacterial growth
Test result: sterile / non-sterile – rating
Version of ISO 11737-2 and associated sterilisation standard used
Data on measurement uncertainty and repeatability

What is the purpose of the examination?

ISO 11737-2 is used for the following scientific and regulatory purposes:

Validation of the sterilisation process: demonstration that the chosen sterilisation process (EO, irradiation, steam, dry heat, plasma, etc.) sterilises the product under the specified conditions
Maintenance of the sterilisation process: Regular verification of the process’s effectiveness in routine production
MDR compliance: Provision of the necessary data for sterility assurance and sterilisation validation under the EU Medical Devices Regulation (2017/745)
Assessment of the relationship between bioburden and sterility: Evaluation of sterilisation efficacy based on the bioburden data specified in ISO 11737-1
Decisions on reuse and reprocessing: root cause analysis and management of corrective actions in the event of detectable microbial growth in the sterility test
Product development and change management: Assessment of the need for revalidation following changes to packaging, materials, design or sterilisation procedures
Support with regulatory submissions: Use as evidence of sterilisation validation for FDA 510(k), EU MDR and other applications
Support with quality control and audits: Demonstrating the effectiveness of the sterilisation process during inspections by the relevant authorities
Evidence-based approach: Contribution to the comprehensive assurance of product sterility in conjunction with data on bioburden (ISO 11737-1), endotoxins (ISO 11737-3), packaging (ISO 11607) and biocompatibility (ISO 10993)

ISO 11737-2 does not, on its own, prove the sterility of the product; sterility is an expression of probability and is assessed in conjunction with the sterility assurance level (SAL). The standard must be applied in conjunction with data on sterilisation validation and microbial load. Any differences from the sterility tests specified in the Pharmacopoeia must be clarified in accordance with regulatory requirements and the product type.

At NanoTest, we carry out sterility tests in accordance with ISO 11737-2 with scientific rigour, within the framework of our accredited laboratory infrastructure and our quality management system.

Please feel free to contact NanoTest for information on method selection, method suitability and validation, sample preparation, incubation conditions, test schedules and report generation in connection with your sterility tests in accordance with ISO 11737-2.

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