The Importance of Pre-Use Integrity Testing in Sterilizing Filtration

Pre-use integrity testing of sterilizing-grade filters eliminates the potential adverse effects of filter loading on the integrity-test results, allowing unambiguous correlation with the integrity-test specification established during filter-validation studies.
May 01, 2007
Volume 2007 Supplement, Issue 2

A septically produced liquid pharmaceutical and biopharmaceutical products are usually sterilized by filtration. The filtration process must be validated to ensure that it is capable of removing all microorganisms from the product. Validation consists of challenging the filter with a suspension of Brevundimonas diminuta and analyzing the filtrate for microorganisms. The filtrate must be sterile.

Filters used in production should be equivalent to the filters used in the bacterial-challenge validation studies. Because actual production filters cannot undergo bacterial-challenge testing, integrity testing is performed to demonstrate bacterial-retention equivalence. If the integrity-test values obtained for the production filters are equivalent to those obtained for the filters successfully passing bacterial-challenge testing, then it is assumed that the filters have the same bacterial-retention properties and that the filtered pharmaceutical product is, therefore, sterile.

Integrity testing for the hydrophilic filters used in pharmaceutical production relies on the measurement of gas flow through wetted membranes. This flow can be classified as diffusive or bulk and is sometimes a combination of both. Fick's Law of Diffusion shows that diffusion of the test gas through the liquid-filled pores in the membrane is a function of the diffusion constant and the solubility of the test gas in the liquid at the test temperature, the pressure differential of the test gas across the membrane, the thickness of the liquid layer, and the area and porosity of the membrane (1). Diffusion is not directly related to pore size although, as will be shown later, there is an indirect correlation. Bulk flow occurs when the test gas flows through the nonwetted or empty pores of the membrane. Open pores occur because the filter membrane has been incompletely wetted or because the bubble point of the membrane has been exceeded. Bulk flow primarily is a function of the size and number of the open pores, the thickness of the membrane, and the pressure differential of the test gas across the membrane at the test temperature.

Gas flow through wetted membranes

Figure 1
As indicated previously, gas flow through wetted membranes is diffusive, bulk, or a combination of both. Figure 1 shows the relationship between gas flow and differential pressure for a typical membrane filter. Similarly, because it is an arithmetic function, pure bulk flow is a straight line whose slope is determined by the size and number of open pores, assuming all other variables are held constant.

The knee area of the curve is where the influence of the bubble point is manifested. Here, the largest pores of the filter become unblocked as the applied pressure overcomes the capillary forces within those pores, and bulk flow begins to increase the slope of the curve. It is also in this region that diffusive flow begins to increase because the thickness of the liquid trapped in the largest pores begins to decrease as a result of the increasing pressure differential. Therefore, within the knee area of the curve a complex relationship exists between diffusive and bulk flow, both influenced by the pore structure and pore-size distribution.

Filter validation

Figure 2
Validating the bacterial-retention capabilities of sterilizing-grade filters involves challenging the filters with a suspension of B. diminuta (ATCC 19146) at a level of at least 1 × 107 CFU/cm2 of filter area, resulting in a sterile filtrate. Specific details and options may be found in "PDA Technical Report No. 26" (2). The integrity-test values of the filters used in the bacterial challenge studies must be known to form the basis for ensuring the filters used in production have equivalent bacterial-retention capabilities. In addition to the integrity-test values, the manufacturing processes and quality-assurance systems of the filter manufacturer must be adequate to ensure consistency of the filters within and between each manufactured lot with respect to nominal pore size, pore-size distribution, and membrane thickness and area.

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