Pre-Use/Post-Sterilization Integrity Testing of Sterilizing Grade Filter – Is the Post-Use Test Sufficient?

Introduction

Sterilizing grade filtration has been reliably used for decades and increasingly so, as more medicinal products are large molecule based, which requires cold sterilization, i.e. filtration. The dependability of this critical aseptic processing step improved with new filter materials, designs and the adoption of filters to single-use process technology to a functionally closed unit operation. Furthermore, process validation undertakings, which evaluate the performance of a sterilizing grade filter under process conditions support the assurance that the filter performs as specified and a sterile filtrate is obtained.^1-3 The process validated state, though, can only be assured by routinely verifying that the sterilizing grade membrane filter is flawless, using non-destructive integrity tests, like Bubble Point, diffusive flow or pressure decay. These tests have been used successfully since the first regulatory requests in the mid 1970s. The integrity test can be performed pre-use/presterilization, pre-use/post-sterilization and post-use, use meaning the actual filtration process, whereby the ratio of the actual integrity test employment varies (Table 1). The post-use test being a regulatory demand is obligatory.

Table 1. Estimation of the integrity test employment ratio

The integrity test of a sterilizing grade filter has to be and most commonly is performed after the filtration process (post-use). Some filter users test the integrity before the filtration process and before the filter is sterilized. Exceptionally rare are integrity tests pre-use/ post-sterilization, as such a test would require downstream, filtrate side manipulation and therefore can be considered precarious and accompanied with a higher process risk. Therefore, most regulatory authorities request the test of the integrity of a sterilizing grade filter after the filtration, post-use, but only recommend an integrity test preuse, without specifying whether pre- or post-sterilization.^2-5 However, EU Annex 1,⁶ paragraph 113 state with the same wording: “The integrity of the sterilised filter should be verified before use and should be confirmed immediately after use by an appropriate method such as bubble point, diffusive flow or pressure hold test….” The paragraph, in its first sentence, recommends the rare and, potentially risk attached preuse/post-sterilization integrity test.

Multiple publications, presentations and position papers^7-9 pointed out the elevation of risks, when using the pre-use/post sterilization test. However, neither the wording of the EU Annex 1 paragraph, nor the current Q&A document¹⁰ have changed. Worst, the pre-use/ post sterilization integrity test has been enforced in some instances, causing a perfectly safe running process being modified to a high complexity and risk attached process.

To determine the motivation of the enforcement of the pre-use/ post sterilization integrity test multiple discussions were held and it seems that the main argument for the need to perform a pre-use/post sterilization integrity test is the belief a flawed sterilizing grade filter pre-use could pass the post-use test, since the flaw could be covered by the separated contaminants from the fluid stream. Such a statement can also be found in the Q&A document,10 which states: “The filtersterilisation process may be physically stressful for the filter. For example, high temperatures during the process may cause the filter to distort, potentially leading to fluid pathways that allow the passage of particles greater than 0.2 µm in size. The performance of a filter can improve with use, as particles begin to block individual pathways and remove larger pathways that smaller particles could successfully navigate. For these reasons, filters should be tested both before use but after sterilisation and again after use.”

This paper will analyze the possibility and show test results of flawed filter pre-use and post-use.

Reviewing the Theoretical Failure Claims

Foremost, pre-use/post-sterilization integrity testing is not a common practice, but rather a rare event. This means that most sterilizing grade filtration applications perform very successfully, processing a sterile filtrate. Incidences of post-use filter failure were detected and the filtered product either discarded or reprocessed.

Sterilizing grade filtration systems require to be sterilized; either presterilized when assembled to a containment bag or sterilized in-situ after assembly into the process or filter housing. The pre-sterilization process, commonly performed by gamma irradiation, is highly qualified by the vendor of the single-use assembly and long-term validation studies have shown that filter systems are not at risk to be damaged by this process. Yet, when the membrane filter is installed into a stainlesssteel filter housing and in-situ steam sterilized, filters may experience higher thermal and mechanical stresses, as mentioned in the Q&A document.¹⁰ Therefore, steam sterilization qualification is a necessity, not only to determine the sterilization efficacy, but also whether the sterilization process stays within the maximum allowable operating parameters given by the filter manufacturer. If such qualification has not been performed or performed poorly or the end-user has not been trained to perform such steam sterilization process, the filter could be damaged. Such damage can cause a flaw, which may allow microbial penetration to the filtrate side. For this reason, post-use integrity testing is mandatory as it allows the detection of such flaws.

There are two theories, though, which claim that a post-use integrity test would not be able to determine a flawed filter. One hypothesizes that a membrane or parts of the pore structure widens due to the steam sterilization temperatures and pressures, the enlargement allows organisms to penetrate the membrane during the filtration process, but also during the process the membrane structure repels back to its original status and therefore the widening cannot be found during the post-use test. The other theory is that the membrane enlargement or a minor flaw in the filter matrix is blocked by the contaminants separated during the filtration process.¹⁰

Test 1. Pre- and post-use integrity test blockage trials with heterogeneous double layer Polyethersulfon 0.2 µm

Analyzing the Likelihood of Occurrence

The first theory speculates that the inline steam sterilization process may enlarge the pore structure, organisms penetrate through the enlarge pore structure, followed by the membrane shrinking back into its original state during the filtration process. Filter membranes, being polymeric materials, are commonly stringently qualified by the filter’s manufacturer. One part of the qualification process is testing the membrane robustness, not just the thermal but also the mechanical robustness. Pulsation tests, up to 20,000 pulsations, at a differential pressure of up to 70 psi (5 bar) are run to determine whether the membrane breaks. In addition, these filter units are subjected to multiple steam sterilization cycles, the integrity test before and after, as well as the bacteria challenge test after the last steaming cycle. Again, results have shown that the membranes can withstand the thermal and pressure conditions, when utilized within the given maximum allowable operating parameters. Neither the pulsation trials nor the steaming cycles have created an enlarged pore structure with any of the commercially available membranes. Filter manufacturers, producing robust, commercially available filter membranes, performing such tests frequently, can attest to these results and often have these documented in their qualification documentation. Decade long experiences also showed that a membrane, which showed a small flaw did not shrink back to its original state, but when a flaw occurred, it has been catastrophic and lasting.^7,11

The hypothesis that an enlarged pore structure or minor flaw within a pleated sterilizing grade filter will be covered by contaminants so that the post-use integrity test cannot detect it, has not been experienced, according to discussions with filter manufacturers.^12,13 Filter manufacturers have performed thousands of bacteria challenge tests, subjecting filter units to 107 organisms per square centimeter, which represents a very high contamination level in the feed stream.¹⁴ Typically, the integrity is tested before the bacteria challenge and post challenge. Reviewed results showed that filters, which failed the integrity test before being subjected to the high bioburden challenge test have also failed the integrity test post challenge. The revision results showed that the post-use integrity test is sensitive enough to detect any flawed filter, even when exposed to a severe contamination level. It has to be pointed out that one typically does not find such high contamination levels within routine filtration applications.

To further confirm the stated and gain additional assurance that the post-use integrity test does expose a flawed filter, no matter what blockage of the membrane, tests were performed at various blocking rates with 0.2 micron filters, some which just failed or passed the preuse integrity test. Important to notice, what shift would be seen in the test result before and after.

Method Description and Results

Multiple pleated 10” filter cartridges 0.2 micron rated with heterogeneous double layer Polyethersulfon and Cellulose acetate) membranes were pre-use integrity tested using the diffusive flow test, afterwards blocked to various degrees and then postuse tested. The filters were wetted with water at 20 °C and integrity tested, diffusive flow for 0.2 micron rated filters. After the integrity test the filters were subjected to a contaminated test fluid at a differential pressure of 4.3 psi to block the filters to various degrees; 25%, 50%, 75% and 95%. After the blocking of the filters, a post-use diffusive flow integrity test was performed for 0.2 micron rated filters. The integrity tests were performed with an automatic integrity tester.

The diffusive flow data of the two different 0.2 micron membrane materials measured in the described trials showed that the post-use test would detect a failed filter independent from the rate of blockage. Even at the highest rate of blockage, 95%, the post-use test would detect a flawed filter. Typically though one would not find such high blockage rate in pharmaceutical filtration applications, therefore the tests represent a worst case scenario.

Figure 1. Test limits for 10” cellulose acetate 0.2 micron are: maximum allowable diffusive flow is 18 ml/min (red line) at 2.5 bar test pressure.

Conclusion and Recommendations

The current reasoning for the enforcement of the pre-use/post sterilization integrity test is the hypothesis, that a slightly flawed filter or a membrane with an enlarged pore structure may be healed during the filtration process by blocking the flaw or the enlarged pore structure, so the post-use test would be unable to detect the flaw. Test data, collected over decades, of integrity tests performed before and after a highly contaminating bacteria challenge test showed that filter do not restore. Furthermore, the discussed test data showed similar results and none of the flawed filters passed the post-use integrity test at various blockage rates, 95% being the most severe.

Figure 2.

Ultimately, the tests showed that the hypothesis of a post-use integrity test not being able to detect a flawed filter after filtration, due to filter plugin, is not valid. Furthermore, based on the robust design and qualification of the commercially available filter membranes concerns related to pore size enlargement as a result of the steam sterilization process followed by subsequent shrinkage back to their original state are not warranted.

Based on these findings, the use of a pre-use/post sterilization integrity test that increases the complexity of the manufacturing process, reduces the level of sterility assurance, and increases the risk to the patient is highly discouraged.

References

1. PDA Technical Report 26, Liquid Sterilizing Filtration, Parenteral Drug Association, Bethesda, MD, 2008
2. Food and Drug Administration (FDA), Guideline on Sterile Drug Products Produced by Aseptic Processing, Division of Manufacturing and Product Quality, Office of Compliance, Center for Drugs and Biologics, Rockville, MD, 2004
3. ISO 13408-2:2003(E), Aseptic processing of health care products –Part 2: Filtration, ISO copyright office, Geneva, 2003
4. Ministry of Health, Labour and Welfare (MHLW), Sterile Drug Products Produced by Aseptic Processing, Tokyo, 2005
5. Pharmaceutical Inspection Convention (PIC/S), Recommendation on the Validation Of Aseptic Processes, Geneva, PI 007-2, 2004
6. EudraLex Volume 4, EU Guidelines to Good Manufacturing Practice Medicinal Products for Human and Veterinary Use, Annex 1, Manufacture of Sterile Medicinal Products, Brussels, 2008
7. Jornitz, M.W. and Meltzer, T.H., Pre-use/poststerilization Integrity testing of Sterilizing Grade Filter: The Need for Risk Assessment, American Pharmaceutical Review, Vol. 14, Issue 5, 2011
8. PDA Pre Use/Post-Sterilization Integrity Test Task Force: Karen Bartel , Hal Baseman, Gabriele Gori, Maik Jornitz, Richard Levy, Hemisha Ly, Russel Madsen, Michiel Rook and Sue Schniepp, PDA Position Paper, Pre-use/ Post-sterilization Integrity Testing of Sterilizing Grade Filters, Vol. 66, No. 5, September–October 2012
9. Ly Hemisha, Presentation: STERILE FILTRATION: Clarification Proposal Annex 1 Paragraph 113: Focus on Pre-use/Post-sterilization Integrity Testing, PDA Meeting, Dublin, July 10th, 2011
10. EU GMP Guide Annexes - Supplementary Requirements - Annex 1 Manufacture of Sterile Medicinal Products 1. Question (H+V June 2007): How should the integrity of sterilising filters be verified?
11. Martin, J., Current Topics in Sterilizing Filtration, ISPE Tampa Conference, Aseptic Processing Course, 2008
12. Thomas, P., Filtration: Debating Post-sterilization Testing, Pharmaceutical Manufacturing, May 2011
13. PDA IG12 Filtration Interest Group Presentation by Millipore, Pall and Sartorius, PDA Annual Meeting Phoenix, April 16-18, 2012
14. ASTM, Standard F838-15, Standard Test Method for Determining Bacterial Retention of Membrane Filters Utilized for Liquid Filtration, American Society for Testing and Materials, West Conshohocken, PA, 1983, Revised 1988, 2005, 2015