Effective sterilization procedures are essential to medical device safety. Rigorous testing is required to confirm that medical devices and biological products are free of harmful contaminants and to isolate and identify any contaminants detected. The FDA has established requirements for acceptable sterility assurance programs. By definition, a sterile material contains no viable organisms. Two sterility testing methods are commonly used:

In the direct inoculation method, the final product is introduced into two types of media and incubated at two temperatures for 14 days. Turbidity indicates contamination, and the contaminant must be identified. If neither medium becomes turbid, a bacteriostasis/fungistasis test is performed to confirm the result. A negative control is included to verify test validity.

In the membrane filtration method, the product passes through a pre-rinsed sterile size-exclusion filter that retains bacteria and fungi. The filter is then divided into two portions, each inoculated into a different medium. The media are monitored for turbidity over 14 days. If turbidity develops, the organism must be isolated and identified. Sterile test articles must also undergo bacteriostasis/fungistasis testing. A negative control is included to verify test validity.

For membrane filtration, the product is passed through a pre-rinsed sterile size-exclusion filter that captures bacteria and fungi. The filter is split into two sections, and each is placed in a different medium. Both media are observed for turbidity for 14 days. Any resulting growth must be isolated and identified. Sterile test articles also require bacteriostasis/fungistasis testing, and a negative control is used to confirm assay validity.

The United States Pharmacopeia and 21 CFR 610.12 recommend two media for both direct inoculation and membrane filtration. With either method, the test article or membrane is incubated in the two media at different temperatures for 14 days.

Sterility Assurance and Bacteriostasis/Fungistasis Testing

Periodic bacteriostasis/fungistasis testing is essential to prevent false-negative sterility test results caused by inhibitory substances in the test article. The test uses six representative bacterial and fungal organisms to confirm that the test article does not prevent or inhibit their growth in the culture medium.