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Assessing the integrity of filter membranes is critical for maintaining sterile filtration operations in laboratories.
Ensuring the performance of sterility filtration is essential in research and pharmaceutical laboratory operations. Many processes demand that the filtrate is free of biological contaminants or else risk downstream contamination, equipment failure, or even human health concerns. Sterility performance of a filter must be tested to validate that it performs as designed.
Environmental and workflow factors, such as handling of filters, particle load, pressure and temperature fluctuations, and chemical exposure steps, can cause wear to filter media. A sterilizing filter can incur damage in the form of membrane blockage, cracks, and changes to pore structure during use. While filter blockages are usually easily detected, other damages may be more difficult to recognize.
Integrity tests are the manufacturer’s and end user’s method to confirm the structural soundness and performance of a sterility filter before and after use as a way to ensure the filter assembly remained integral.
In this article, we review the two types of integrity tests—destructive and nondestructive—that are commonly used by the industry to validate sterility filter performance. It explains the different testing methodologies and the reasons why they are critical to manage laboratory risks.
Destructive integrity testing is performed by the filter manufacturer during product development and/or as part of manufacturing quality controls. The goal is to ensure the filter will function according to the manufacturer’s sterility claims and ensure there are no leaks or defects in the product. These tests are typically conducted on new laboratory filters before a new batch. For sterility filters, the integrity test involves introducing a microorganism upstream and ensuring zero organisms are recovered post filter.
This type of test is called “destructive” because once performed, the filter can no longer be used. Whenever the filter is intentionally exposed to a microorganism to test its performance, it cannot return to service and must be destroyed.
Another goal of destructive testing is to provide a correlation to a nondestructive test that can be performed by an end user. Validating a nondestructive integrity test value to a destructive test result ensures that when the end user performs the test before and after using the filter, the results will match the manufacturer’s claims.
Laboratory end users typically rely on nondestructive tests to validate the performance of their sterility filtration. In reviewing the types of nondestructive testing, it’s important to understand that there are two classes of membrane filtration. Hydrophilic membranes are water loving while hydrophobic membranes are water repellant. The type of membrane used will determine the appropriate nondestructive integrity test.
There are several nondestructive tests commonly used to test the integrity of hydrophilic membranes: the bubble- point test, the forward-flow test, and the pressure-hold or pressure-decay test.
Bubble-point test. This test is the most popular for small filtration devices, primarily because these filters do not usually have the solution volume needed to conduct the other tests. The bubble-point test can be performed manually or with an automated device that measures the minimum bubble point value. It’s important to note that the manual test requires well-trained operators that can differentiate between a stream of bubbles versus filter media compression. A bubble-point test works by thoroughly wetting the membrane, attaching a pressure gauge upstream, and applying air pressure to the filter up to the point of the minimum bubble-point. If no bubbles are observed up to the specified bubble-point value, the filter is considered integral.
Forward-flow test. This test is more accurate than the bubble-point method. While it can be used on small, hydrophilic membrane filters, it is commonly performed on larger devices. The test injects a constant amount of air at a specific flow rate through the wetted filter. The volume of air displaced from the upstream side to the downstream side is continuously measured. If the volume maintains a rate below a certain limit, the filter is integral.
Pressure-hold/pressure-decay test. This test pressurizes the upstream side of the wetted filter. When the pressurization is stopped, the amount of pressure decay that occurs over time is measured. If it is below a certain limit, the filter is integral.
These three methods can also be utilized to test hydrophobic membrane integrity. In addition, laboratory staff can perform a fourth method: the water breakthrough/water intrusion test.
Water breakthrough/intrusion test. As we have said, hydrophobic membranes repel water. If a small volume of water is pushed through a hydrophobic filter at a specific pressure (2 bar), there should not be a liquid eluting or passing through the hydrophobic membrane. If the membrane holds back the water for 10 to 15 seconds, the filter is integral and ready for use. This test is popular because it uses a small amount of water and is quick and easy to perform.
Bubble-point and forward-flow tests. Conducting one of these tests requires breaking the surface tension of the hydrophobic membrane. To do this, typically 100% isopropyl alcohol (IPA), or IPA/water solution is used to thoroughly wet the membrane. Then the previously discussed bubble-point method can be applied to determine the point when a stream of bubbles is visible coming out of the filter. Alternatively, the forward-flow test measures the volume of solution passing through the filter. The disadvantage of using both of these methods is that the filter needs to be dried to remove all the water and solvent prior to use. Because of this, the water intrusion/ breakthrough test is preferred for hydrophobic filters due to achieving test results more quickly.
Several factors should be considered when conducting tests.
Wetting fluids and gas types. The selection of wetting fluid types for non-destructive testing is often overlooked by laboratory end users. Water is typically used to test a hydrophilic filter. If testing a hydrophobic membrane, the filter must be wetted with 100% ethanol or IPA, or a blend of IPA and water example 70/30%, respectively.
The type of testing gas also must be considered. The two popular gases are air, or if there is concern about oxidation, nitrogen. Other gases are not used due to health and safety concerns or the fact they are not compressible.
Temperature control. The other factor that can be overlooked during integrity testing is temperature control. Temperature has a strong impact on gas expansion. If the temperature of the air or nitrogen is elevated during integrity testing, the result will be higher diffusive flow that may skew the test results. If the gas is too cold, it may also alter the testing outcome. It is critically important to maintain ambient temperature throughout the testing process to get the most accurate results. In cases where the end user wants to test at a higher temperature, the filter manufacturer should be queried for values calculated based on that temperature.
Most laboratories are doing an excellent job conducting non-destructive tests to validate their sterility filter’s performance. If a lab is not doing this, it is highly recommended to implement this testing to reduce risk and ensure repeatability of their work.
The constant changeover in laboratory personnel places a need for continuous training, particularly on manual bubble-point tests, because the results can be subjective, and their accuracy is based on the user’s technical skills. For example, when beginning the test, the filter device is pressurized, which results in initial compression. Trapped gas in the filter will escape and those bubbles can be mistaken for a minimum bubble-point value.
Another common error is over-pressurizing the filter. Using the example of a filter where the minimum bubble-point value is 20 psi, the pressure reaches 20 psi, and the user does not see a stream of bubbles. The tester may continue to raise the pressure until bubbles are seen at 35 psi; however, this is an incorrect test procedure. If the minimum bubble-value point value is 20 psi, there is no need to increase the pressure because the membrane has passed the test. On the other hand, if a stream of bubbles were visible at 17 psi, that is below the minimum bubble-point value and the membrane will have failed the test.
Over-pressurizing the membrane in a bubble-point test can result in filter damage and invalid data, particularly after it has been used and may be clogged. Best practice calls for performing the test at a slow and steady rate to make sure the correct amount of pressure is applied.
Another trend is the growth of automated integrity testing. Although most laboratory filtration products are validated using the manual bubble-point method, the trend is towards more automated procedures that require less time and remove human error while achieving higher productivity.
The goal of filtration integrity testing is to ensure performance and reduce risk. Manufacturers perform destructive tests to validate the claims of their filter’s performance. But once these products exit the companies’ shipping docks, they leave controlled environments. There are many conditions and events between the manufacturer and the end user that can affect a filter’s sterility performance. Filter can sustain damage during shipment and installation that can go undetected without integrity testing the filter before use.
When in use, the solutions that enter the filter may have cell debris and contaminants that can affect the researcher’s work or downstream process. The scientist or lab technician expects that the liquid passing through the filter is a sterile solution. Failure to perform a filter integrity test can put the laboratory’s work and even staff at risk.
The only way the laboratory can guarantee the performance of sterility filtration is to conduct integrity tests on the products, before and after use.
Joseph Baaklini is the director of the Scientific and Laboratory Services Global Technical Support group for Pall Corporation and leads the Pall Laboratory and Food & Beverage technical division globally.
Vol. 44, No. 11
When referring to this article, please cite it as J. Baaklini, “Understanding the Critical Role of Integrity Testing in Laboratory Sterility Filtration,” Pharmaceutical Technology 44 (11) 2020.