Biopharmaceutical manufacturers and engineers are implementing filter integrity testing procedures that incorporate single-use systems both upstream and downstream in the manufacturing process to improve their processes and reduce risks associated with product contamination. This article addresses how single-use components can be incorporated into manufacturers' filter integrity testing procedures. Specific examples include post filtration integrity testing for a media and buffer, a pre-filtration addition point (application), and filtrations between purification and formulation steps.
Overview of Filter Integrity Tests
Filter integrity testing verifies that the filters used in a bioprocess are functioning properly and are capable of removing a minimum particle size from filtrate. These tests can be conducted pre- or post-filtration, or both, and are primarily non-destructive. The standard non-destructive testing methods are bubble-point, gas diffusion, and pressure decay tests.
The first step in all three tests requires that the filter membrane be thoroughly wetted with a flush solution. During testing, the filter inlet is sealed while controlled pressure, typically in the form of compressed air or other gas, is applied to the proximal side of the filter membrane. Excess filtrate is allowed to exit the filter outlet to a flush bag or collection vessel.
In the bubble-point test, pressure is gradually increased until bubbles are observed exiting the filter outlet. The pressure at which the bubbles are observed indicates whether or not the filter is functioning correctly. This is the simplest test to perform and requires the highest pressure.
The gas diffusion test uses lower pressure and measures gas as it diffuses through a filter membrane to determine integrity. During the test, a calibrated gas flow meter is attached to the filter outlet, potentially compromising sterility. Both the bubble-point and gas diffusion tests typically are conducted post-filtration.
Pressure decay tests do not compromise sterility, are performed at low pressure, and are widely used for in-process testing. A calibrated pressure gauge is used on the proximal side of the filter rather than using visual observations or an outlet flow meter as in the other tests. Filter integrity is determined by the rate of pressure decay across the filter; excessive pressure decay indicates failure.
In addition to non-destructive testing, destructive testing can be conducted post-filtration for the final formulation and fill process. Rigorous bacterial challenge testing passes a known quantity of challenge bacterium into a filter. The resulting filtrate is aseptically transferred to growth-promoting medium to confirm that all bacterial cells were retained in the filter. Bacterial growth indicates filter failure.