Standard Historical Last Updated: Mar 22, 2023 Track Document
ASTM E3251-20

Standard Test Method for Microbial Ingress Testing on Single-Use Systems

Standard Test Method for Microbial Ingress Testing on Single-Use Systems E3251-20 ASTM|E3251-20|en-US Standard Test Method for Microbial Ingress Testing on Single-Use Systems Standard new BOS Vol. 14.01 Committee E55
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Significance and Use

4.1 Single-use systems (SUSs) used for biopharmaceutical manufacturing must maintain sterility and product quality of the fluid inside. Such articles or systems should therefore be validated as providing an effective barrier against microbial ingress. The microbial barrier properties of a SUS may be demonstrated using deterministic physical tests that have been correlated to microbial integrity. Two test methods (aerosol exposure and immersion exposure) are described that can be used to demonstrate microbial integrity of a SUS or determine the MALL, the maximum defect size that does not allow microbial ingress, into a SUS.

4.2 It is important to note that the results of microbial ingress tests are heavily dependent on the conditions under which the test is performed and are not suitable for routine checking of a SUS due to the test’s destructive nature.

4.2.1 Any size defect may be forced to fail under sufficiently aggressive conditions (including a large enough sample size, high differential pressure, or high hydrostatic pressure, for example) that would not ordinarily reflect normal use conditions. Thus, it is necessary to clearly define the relevant conditions for a test through a risk assessment of both the actual SUS claims and its final use (Practice E3244). Once that is established, the size of defect that can be detected under those conditions can be determined, if required, using defined defects.

4.2.2 “Relevant conditions” refers to worse-case actual use conditions but does not mean that a SUS must be tested under theoretically absolute (extreme) “worst-case” conditions.

4.2.3 Testing may be performed on individual components or entire systems. Considerations for defining “relevant conditions” and testing design should be based on a risk assessment for the SUS intended use and should include: A channel created by a defect or breach through the film thickness or through a seam or connection which must be filled with liquid to allow microbial passage.5, 6 Factors that could influence liquid filling of a channel, including a liquid’s viscosity, defect size and type, plastic materials and pressure applied inside the SUS. Rationale for selecting a defect type should be based on the probable type of defect(s) that could occur during the SUS life cycle Rationale for selection of defect sizes should be based on a deterministic physical testing method (detection limit) Consideration of pressure(s) differential applied during testing to simulate conditions that a SUS may be subjected to during actual use conditions (Practice E3244).

4.3 The selection of challenge microorganism and minimum target challenge concentration should be based on a risk assessment, justified, and validated, as necessary, for the limit of detection. A minimum of 106 CFU/cm2 surface area (aerosol) or 106 CFU/mL (liquid immersion) is typically used (ISO 15747 and Aliaskarisohi7).

4.4 SUS test articles bearing calibrated defects may be produced and tested to allow either the determination of the minimum defect size that can be detected by a microbial test method under given conditions (for example, microbial ingress) or to determine the MALL of SUSs under use-case conditions (for example, aerosol test).

4.4.1 If the test objective is to determine the MALL and demonstrate correlation between physical integrity test sensitivity and microbial ingress, selection of the artificial defect (laser-drilled hole, capillary, copper wire) should be based on the most probable type of defect that could occur during the SUS’s life cycle.

4.4.2 The selection of defect sizes should be based on the expected transition from ingress to no ingress under the SUS’s intended use-case conditions, alternatively, worst-case conditions can be selected. As described in the Practice E3244, a typical range is from 1 µm to 100 µm. The defect sizes should be calibrated by a defined method.

4.4.3 One approach for determining the MALL of a SUS film material is to test single-use film coupons with calibrated defects, in holders. This enables higher throughput testing; however, using coupons as test articles may not represent a scale-down model of an entire SUS.

4.4.4 Another approach is to validate the test method on alternative container-like vials. The principle remains the same. The alternative container must be able to hold the minimum size defect.

4.5 These procedures should be conducted in a microbiological laboratory by trained personnel. It is assumed that basic microbiological equipment and supplies for conducting routine microbiological manipulations (for example, standard plate counts, autoclave sterilization, etc.) are available.


1.1 The microbial test method outlined in this document applies to microbial ingress risk assessment of a single-use system (SUS) or its individual components that require integrity testing either by the assembly supplier or the end user of the assembly based on a potential risk of a breach to the product or manufacturing process.

1.2 The aim of microbial ingress testing of sterile SUSs used in biopharmaceutical manufacturing is two-fold:

1.2.1 Firstly, it is used to evaluate the ability of a SUS fluid path to remain sterile after a SUS has been challenged by microbial exposure. Microbial exposure is achieved either by directly placing a SUS into a container of microbial challenge solution, or by delivering an aerosolized microbial challenge onto a SUS that is placed inside a test chamber designed to generate and deliver the aerosol. The choice of the test challenge organism should be justified based on a risk assessment of the SUS and conditions of use.

1.2.2 Additionally, microbial ingress testing can be used to determine the maximum allowable leakage limit (MALL) that does not allow microbial ingress under specific test conditions. The defect size that can be detected by specific physical integrity testing methods can be correlated to this MALL in order to claim microbial integrity. Test articles bearing calibrated defects over a range of dimensions, including up to a defect size expected to consistently allow microbial ingress as a positive control (defect-based positive control), may be tested to determine the MALL.

1.3 Both purposes for microbial ingress testing as described in 1.2.1 and 1.2.2 can either be conducted by liquid immersion or aerosol exposure. For the purpose described in 1.2.2, the type of exposure should be determined according to the SUS’s use-case conditions and a risk assessment.

1.4 The method used to create a breach, hole or defect in single-use film or in a SUS test article, as well as the analytical method used to physically characterize the defect size is outside of the scope of this document. The sampling plan for a given test article should be justified with the rationale of sampling size to obtain a statistically meaningful effect (Practice E3244). Determining the appropriate number of SUS test articles will depend on a risk assessment of the SUS and the conditions of its use and is also outside of this document’s scope.

1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

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Book of Standards Volume: 14.01
Developed by Subcommittee: E55.04
Pages: 8
DOI: 10.1520/E3251-20
ICS Code: 07.100.10