Mainly in the pharmaceutical industry, filter integrity testing is an essential step to guarantee the sterility of a fluid (liquid or gas). Integrity tests are also particularly necessary in the beverage and food industries.
What is the primary purpose of integrity testing in filtration?
It consists in checking, before and/or after use, that the filter employed complies with its specifications, including efficiency — particularly for final filters intended to ensure the sterility of a fluid, whether liquid or gas, and therefore filters meeting HIMA or ASTM challenges for the pharmaceutical sector, generally 0.2 or 0.1 µm. Other industries, such as the beverage industry, are also concerned. This makes it possible, for example, to ensure that the filter is not defective due to a membrane rupture.
In the pharmaceutical field, the European GMP Guide – Annex 1 Revision, Effective January 1997 – states that:
Good to know
The integrity of the sterilised filter must be verified before use and confirmed after use by an appropriate test method.
Likewise, the European Guidelines on Good Manufacturing Practice – European GMP, 5th Edition – 1998 – state:
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The integrity of vents and air filters located at critical points must be confirmed after use.
Another example, the Recommendations on the Validation of Aseptic Processes – Pharmaceutical Inspection Co-operation Scheme, September 2001:
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It is important to confirm the integrity of a vent or critical gas filter immediately after filling…
Finally, in the FDA Guide for Inspections of Purified Water Systems:
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A procedure for performing the integrity test of the vent filter on a purified water storage tank must be established to ensure its integrity. Methods exist to test a filter’s integrity in situ.
Which filters are subject to integrity testing
Primarily, sterilising filters. Whether hydrophilic membrane filters for liquids or hydrophobic membrane filters for gases. These may include filters used to ensure product sterility during manufacturing — for example during the addition of product components or injection of gases — as well as tank or container vents used for storage, transport or final packaging.
Types of integrity tests in filtration
Different types of integrity tests exist.
For hydrophilic filters, bubble point and diffusion tests are generally the most commonly used (1).
For hydrophobic membrane filters, testing is usually done using the water intrusion test (2), also called the WIT test.
Summary of the tests:
| TEST | MEDIUM | TEST PURPOSE |
|---|---|---|
| Bubble Point | Air | Pore size |
| Air Diffusion | Air | Integrity |
| Pressure Hold | Air | Integrity |
| Latex Beads | Latex Beads | Integrity |
| Water intrusion | Water | Integrity |
| DOP | Dioctyl Phthalate | Integrity |
Other useful tests in filtration
| TEST | MEDIUM | TEST PURPOSE |
|---|---|---|
| Bacterial challenge | Pseudomonas Diminuta | Sterility |
| Retention capacity | Water + calibrated contaminant | Filtration efficiency |
| Electron microscopy | – | Pore size |
| Resistivity | Water | Filtration efficiency |
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