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
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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
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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.
