It has been shown that hydrodynamic flow is preferentially directed to the larger pores in a filter membrane (4). This means
that hydrodynamic flow directs the suspended particles to the largest pores. However, because the largest pores are far less
numerous than the average-sized pores, particles initially tend to be captured by the many smaller pores. As the smaller pores
become blocked, the hydrodynamic flow directs more of the particles to the larger pores. The larger pores now may become blocked
or partially blocked because of bridging, which is promoted by high levels of particulates competing for the pore openings.
This phenomenon is corroborated by Grant and Zahka (5). They showed that during the filtration of a dilute suspension of silica
particles there was complete retention at the beginning of the filtration, followed by particle passage. The retention then
increased as the filtration continued. Similar results were obtained by Roberts and Velazquez while filtering latex particles
(6). Thomas et al. found that the successive changes affecting a filter's pore-size distribution as a filtration of Pseudomonas aeruginosa progressed showed the larger pores to be less immediately engaged in the retention of particles (7).
Effect of filter loading on integrity-test results
The effects of filter loading on diffusive flow and the bubble point have been evaluated; however, comprehensive studies remain
to be performed, and understanding of these effects is currently incomplete. Trotter et al. showed that the diffusive-flow values at 45 psi began to change at a bacterial cell loading of 1 × 102, while the bubble point remained essentially unchanged until the bacterial cell loading approached 1 x 108 (8).
Particle capture inherently reduces the porosity of the filter, thus decreasing the diffusive flow, and it increases the bubble
point as the largest pores become blocked. Also, the effective thickness of the membrane increases as material accretes on
its surface. These effects have implications with respect to the integrity-test values and their correlation to bacterial
retention. Certainly, porosity reduction and the increase in thickness depend on the feed stream and the properties of the
particles (gel or solid) removed from it.
Filter loading influences diffusive-flow and bubble-point values, although not to the same degree and not in the same sequence
according to published studies. Postfiltration integrity testing evaluates filters that have been subject to an unknown level
of particulate loading; therefore the influence of the particulate loading on the integrity-test results is also unknown.
It is reasonable to assume, however, that as particles are arrested by the membrane, the porosity of the membrane decreases
and the bubble point increases. This assumption has been confirmed experimentally (8). Testing under different conditions
or with different membrane types and configurations could produce different results, however.
Extensive validation testing using multiple lots of product representative of normal production is necessary to determine
the potential effects of filter loading on the postfiltration integrity-test values. In any case, the precise effect of the
filter loading on the postfiltration integrity test cannot be known with certainty; therefore, it is not possible to ensure
that filter loading has not masked a defect that initially may have been present in the filter.
Recommendations and conclusion
Regulatory agencies differ in their positions regarding the need for prefiltration integrity testing of membrane filters used
to sterilize liquid pharmaceutical and biopharmaceutical products. Annex 1 of the EC Guide to Good Manufacturing Practice specifies integrity testing of the sterilized filter before and after use. FDA's 2004 Guidance for Industry, Sterile Drug Products Produced by Aseptic Processing suggests that integrity testing can be performed before processing and that integrity testing should be routinely performed
post-use to detect leaks or defects that may have occurred during the filtration process. The FDA guidance does not take into
account potential changes in the validated prefiltration integrity-test values as a result of filter loading.
Because the validation studies correlating bacterial retention with integrity-test results use prefiltration integrity-test
values, albeit with a safety factor, the only reliable way to ensure that the filters used in production are equivalent to
those used to establish the integrity-test specification is to perform a prefiltration integrity test. The postfiltration
integrity test then serves its intended function: to ensure the filter has not been damaged during the sterilization or filtration