Sterility Testing
Sterility testing is an essential requirement in Good Manufacturing Practices (GMP) microbiology to ensure that sterile products are free from viable microorganisms before they are released for patient use or administration. Given the critical nature of sterility, testing methods must be highly accurate and reliable to confirm that medical devices, pharmaceutical products, formulations, tissue materials, and other products that are intended to be sterile meet the necessary standards for safety and quality. This testing is vital to prevent contamination and protect patients from potential infections caused by microorganisms in products that claim to be sterile or free from viable organisms.
Overview
Pharmaceutical Sterility Testing Methods
Compendial methods for the sterility testing of pharmaceutical products require samples to be cultured in two separate media. Two different types of culture media are used in sterility testing to promote the growth of residual anaerobes, as well as aerobes and fungi:
- Fluid Thioglycolate Medium (FTM): Used to culture anaerobic and some aerobic bacteria.
- Soybean Casein Digest Medium (SCDM): Used to culture fungi and aerobic bacteria.
Samples are incubated for 14 days at 32.5 °C and 22.5 °C, respectively, prior to examination. Any turbidity in the culture media may indicate growth and must be investigated. There are two recommended methods of sterility testing for pharmaceuticals: membrane filtration and direct inoculation.
Membrane Filtration Sterility Testing
Membrane filtration sterility testing is the regulatory method of choice for filterable pharmaceutical products, as cited in the USP <71>, EU Pharmacopoeia <2.6.1>, and JP Pharmacopoeia <4.06>. This method involves:
- Passing samples through a 0.45 µm membrane filter in a filtration canister.
- Adding culture medium for incubation.
- Increased sensitivity as the whole sample, or a composite sample, is passed through a single filter.
- Filtration allows rinsing away components that may cause turbidity or inhibit growth, such as antibiotics or preservatives.
Direct Inoculation Sterility Testing
In direct inoculation, a small volume of sample is removed aseptically from the sample unit and inoculated directly into a suitable volume of growth medium before incubation. This method involves:
- Inoculating small volumes of product into the culture medium, limiting test sensitivity.
- Difficulty in detecting turbidity from microbial growth if the sample appears cloudy or turbid after inoculation.
- Neutralizing antimicrobial products to prevent growth inhibition.
Sterility Testing Methods for Medical Devices
Direct transfer sterility testing is recommended for the sterility testing of medical devices. The device to be tested is in direct contact with the test media throughout the incubation period, during which any microorganism in or on the device will grow and proliferate. For products with hollow tubes, such as transfusion and infusion assemblies, product flush sterility testing is preferred. This method includes:
- Flushing the product lumen with a rinsing fluid.
- Membrane filtering the elute.
- Placing the filtered elute in a suitable media for incubation.
Sterility Testing Workflow
The following steps outline the sterility testing workflow for membrane filtration:
- Test Preparation: Place the membrane filtration device tubing into the pump.
- Filter Prewetting: Prewet the membrane filtration device to optimize filtration and minimize product binding to the filter.
- Sample Filtration: Filter an equal amount of product into both canisters. Microorganisms are retained by a 0.45 µm pore size filter.
- Rinsing: Rinse all inhibiting compounds using a suitable rinse solution(s).
- Culture Media Filling: Fill the membrane filtration devices with TSB and FTM sterile culture media.
- Incubation Time: Incubate for 14 days.
- Final Test Results: Check for visual turbidity or no turbidity to determine the results.