Madeline Kennedy, Microbiologist III
The sampling quantities defined in USP <71> Sterility Tests are determined based upon statistical probabilities of identifying contamination, should it be present, given various article and batch sizes. The specific direction to follow is found in tables 2 and 3 in the chapter. Table 2 defines the minimum volume per test article to be inoculated into each sterility media type. Since there are two types of media used during a USP <71> sterility test, tryptic soy broth (TSB) and fluid thioglycolate medium (FTG or FTM), the quantity noted in the table represents half of the per-article test volume needed to complete a USP <71> compliant sterility test. Table 3 defines the minimum number of articles of a final product to be tested based on the batch size of the lot being assessed. Complying with the sampling guidelines in USP <71> provides the recommended level of stringency for sterility testing of compounded pharmaceutical products and is the minimum acceptable quantity to test to ensure confidence in a “Sterile” result.
Below are two examples of determining the appropriate sterility test volumes according to USP <71> tables 2 and 3:
· A batch of 400 vials, filled with 50mL each – According to Table 2, this article size requires 20mL from each vial to be inoculated into each sterility media type (total of 40mL per article); and for this batch size, 10 containers would need to be tested. As such, the USP <71> compliant test volume for this batch would be 400mL.
· A batch of 600 syringes, filled with 1mL each – According to Table 2, this article size requires the whole contents of each container be inoculated into each sterility media type (which will double the required number of containers required to be tested according to Table 3); and for this batch size, 24 containers (double the 12 noted in Table 3 due to the fill volume) would need to be tested. As such, the USP <71> compliant test volume for this batch would be 24mL.
Additionally, ensuring appropriate volume requirements are met during sterility method suitability testing is equally important. Test volume is an important component of the method suitability demonstration, as any inhibitory or antimicrobial properties present in the product may variably impact the ability to recover the method suitability test microorganisms depending on volume tested. As an example, minimal inhibitory effects may be seen when performing a sterility test on ten 10mL vials (100mL total) of a product. However, if the vial size of the same product is increased to 30mL, the sterility test volume would increase from 100mL to 300mL, and inhibitory properties may then be present. In that, or any, case where container or batch size is changed, it is important to ensure appropriate sterility method suitability testing is re-performed if necessary. This is due to the increase in active ingredients, inactive ingredients, and preservatives, any of which could potentially impact microorganism growth. Should an ineffective method be used to perform a sterility test on a larger volume, the risk for a false negative test result increases. For potential cost and time savings, ARL Bio Pharma recommends performance of sterility method suitability testing on the largest potential container / batch size combination for a given product. This will ensure the test volume validated covers all subsequent sterility test submissions.
· USP <71> Sterility Tests
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