ISO standards for gamma and electron-beam sterilization use special tests to measure how well a product meets sterilization parameters. It's useful for designers and managers to understand the testing in terms of a validation process. Generating adequate and reliable sterility-test data is an important quality-control issue. But sterility testing is a tedious yet artful process that must be performed by trained and qualified laboratory personnel. Investigating sterility-test failures requires attention to environmental data and many other factors such as training and sample difficulty.

There are typically two ways to test sterility in medical devices. Direct-transfer sterility testing immerses the device in two separate baths of test fluid. The device is incubated for 14 days, and if it has not been adequately sterilized, microorganisms will grow. The second and less common test is called the product-flush sterility test. It's for products that have hollow tubes where immersion is not practical.

Sterility testing is a difficult task that must be designed to eliminate false-positive results. These are generally due to laboratory contamination from the testing environment or technician error. The environment must meet requirements of the United States Pharmacopeial (USP) for levels viable microbial air and surface counts. Growth media used in sterility testing must be meticulously prepared and tested to ensure it can support microbial growth.

The United States Pharmacopeia is a compilation of validated methods and official monographs for pharmaceuticals and medical devices. The USP is broken into sections for Monographs, General Informational Chapters, and General Requirements. General Informational Chapters are not legal requirements. But the Sterility Test (USP Section 71) is categorized under General Requirements. Therefore it is a legal requirement.

The ISO radiation sterilization microbial method (11737-2 1998) describes a sterility test which is a modification for the USP method. This test detects aerobic organisms which have been exposed to sub-lethal sterilization cycles. The method is recommended for validating gamma and electron-beam sterilizations. ISO recommends that the sterility test be validated by using known sterile products.

Environmental concerns

The sterility test environment is described in USP General Informational, Chapter 1211. The environment should be as stringently controlled as a cleanroom. Such a room delivers laminar flow air which has been filtered through microbial retentive HEPA filters. The room is kept at a positive pressure and has specifications for room air changes per hour. An area used for sterility testing should be similar in design to a cleanroom. There should be an anteroom for gowning and a separate area for the actual sterility testing. The testing area should meet ISO Class 5 particulate control requirements (specified in USP chapter 1116).

Along with particulate testing in the environment, the laboratory must test for viable bacterial and fungal organisms. The sterility test technician must be suitably gowned in sterile garments that prevent microbial shedding into the room. The room should be validated in terms of particulate and microbial levels. The laboratory must have a validation and training program for gowning and sterility testing.

Pretest action

Prior to sterility testing, it helps to send a sample to the testing laboratory so it can determine an appropriate testing procedure. Each product should have its own procedural specification for testing. The procedure should describe which items to test (in the case of kits) and indicate the Sample Item Portion (SIP). This is the percentage of the complete product tested. For example, large and cumbersome devices are difficult to test in their entirety. Therefore, the test laboratory will determine a SIP, for example 10%. This number is used in gamma and electron beam dose setting methods. The SIP should be validated by sterility testing.

Combination products have their own challenges. These are medical devices combined with a drug or biologic, which is a product prepared from animal tissue or some other living source. For example, a drug-coated stent. The agency Office of Combination Products would determine which regulatory branch (CDRH, CDER or CBER) is officiating the product. Official USP sterility testing of combination products is required for all sterile drug products. The aseptically applied drug creates the largest challenge to laboratory personnel. Biologics must be aseptically processed and cannot be terminally sterilized by ethylene oxide. In the near future, we will see more biologics that are combination products. When they are sterilized by radiation they are generally handled as medical devices following the ISO 11137 standard. For the most part, stricter pharmaceutical GMPs take precedent over 820 QSR requirements with all combination products.

The USP 71 Sterility Test begins with two qualifying assays. They are the Suitability Test (Growth Promotion Test) and the Validation Test (Bacteriostasis and Fungistasis Test).

The Suitability Test confirms that each lot of growth media used in the sterility-test procedure will support the growth of less than 100 viable microorganisms. If the media cannot support the growth of the indicator organisms, then the test fails. Secondly, a portion of each media lot must be incubated and assessed for sterility according to the incubation parameters (time, temperature) established by the method. If the media is found to be non-sterile, then the test fails.

The Validation Test determines if the test sample will inhibit the growth of microorganisms in the test media. Stasis, in terms of microbiology, is the inability of a microorganism to grow and proliferate in microbiological media. Bacteriostatic media does not necessarily kill bacteria, it simply may retard bacterial growth and proliferation. The Validation Test must be performed on each product prior to and during sterility testing. This test determines if the media volumes are valid for the particular product. Some medical products contain bacteriostatic and fungistatic compounds that may require special procedures and special media for testing. This test is similar to the Suitability Test, however, the product sample is placed in the media along with the microorganisms. Microbial growth in the presence of the test samples is compared to controls without test samples. If microbial growth is present in the sample and control containers, then the test is valid. The next step is actual sterility testing. Suitability, validation, and sterility tests can be performed simultaneously.