Filter Integrity Testing

Sterilizing grade filters require to be tested to assure the filters are integral and fulfill its purpose. Such filter tests are called integrity test and are performed before and after the filtration process.  Sterilizing grade filtration would not be admitted to a process, if the filter would not be integrity tested in the course of the process. This fact is also established in several guidelines, recommending the use of integrity testing, pre- and post filtration. This is not only valid for liquid, but also air filters.

Examples of such guidelines are: 

FDA  "Guideline on Sterile Drug Products Produced by Aseptic Processing" (2004),
"Normally, integrity testing of the filter is performed after the filter unit is assembled and prior to use. More importantly however, such testing should be conducted after the filter is used in order to detect any filter leaks or perforations that may have occurred during filtration."

EudraLex, Volume 4, Annex I (2008)
"113. The integrity of the sterilized filter should be verified before use and should be confirmed immediately after use by an appropriate method such as a bubble point, diffusive flow or pressure hold test. The time taken to filter a known volume of bulk solution and the pressure difference to be used across the filter should be determined during validation and any significant differences from this during routine manufacturing should be noted and investigated. Results of these checks should be included in the batch record. The integrity of critical gas and air vent filters should be confirmed after use. The integrity of other filters should be confirmed at appropriate intervals."

ISO 13408-2 "First edition, 2003-03-15, Aseptic processing of health care products", Part 2: Filtration (2003)
"9.5 The validated physical integrity test of a sterilizing filter shall be conducted after each use without disturbing the filter in its housing. Physical integrity testing of a sterilizing filter in situ should be conducted before use after sterilization where the design of the filtration system permits.
Care should be taken not to compromise the sterility of the filter."

USP (United States Pharmacopoeia) 23, (1995)

Guide to Good Pharmaceutical Manufacturing Practice (Orange Guide, U.K., 1983,)

PDA (Parenteral Drug Association), Technical Report No. 26, "Sterilizing Filtration of Liquids" (March 1998, revised 2008)

Integrity tests, as the Diffusive Flow, Pressure Hold, Bubble Point or Water Intrusion Test, are non-destructive tests, which are correlated to the destructive bacteria challenge test with 107 per square centimeter Brevundimonas diminuta . Derived from these challenge tests specific integrity test limits are established, which are described and documented within the filter manufacturer's literature. The limits are water based, i.e. the integrity test correlations are performed using water as a wetting medium. If a different wetting fluid, respectively filter or membrane configuration, is used, the integrity test limits may vary.  Integrity test measurements depend on the surface area of the filter, the polymer of the membrane, the wetting fluid, the pore size of the membrane and the gas used to perform the test. Wetting fluids may have different surface tensions, which can depress or elevate the Bubble Point pressure. The use of different test gases may elevate the diffusive gas flow. Therefore appropriate filter validation has to determine the appropriate integrity test limits for the individual process, if a process fluid is utlized.

Bubble Point Test
Microporous membranes will fill their pores with wetting fluids by imbibing that fluid in accordance with the laws of capillary rise. The retained fluid can be forced from the filter pores by air pressure applied from the upstream side. The pressure is increased in gradually in increments. At a certain pressure level, liquid will be forced first from the set of largest pores, in keeping with the inverse relationship of the applied air pressure P and the diameter of the pore, d, described in the bubble point equation:

           P  =  4  cos 

where ; is the surface tension of the fluid and ; is the wetting angle; P is the upstream pressure at which the largest pore will be freed of liquid, d is the diameter of the largest pore.
When the wetting fluid is expelled from the largest pore, a bulk gas flow will be detected on the downstream side of the filter system. The Bubble Point measurement determines the pore size of the filter membrane, i.e. the larger the pore the lower the Bubble Point pressure. Therefore filter manufacturers specify the Bubble Point limits as the minimum allowable Bubble Point. During an integrity test the Bubble Point test has to exceed the set minimum Bubble Point.

Diffusion Test
A completely wetted filter membrane provides a liquid layer across which, when a differential pressure is applied, the diffusive airflow occurs in accordance with Fick's law of diffusion. The upstream pressure applied is called test pressure and commonly specified at ~80 % of the Bubble Point pressure. In an experimental elucidation of the factors involved in the process, Reti simplified the integrated form of Fick's law to read

        N = DH (p1 - p2)  

where N is the permeation rate (moles of gas per unit time), D is the diffusivity of the gas in the liquid, H is the solubility coefficient of the gas, L is the thickness of liquid in the membrane (equal to the membrane thickness if the membrane pores are completely filled with liquid), P (p1-p2) is the differential pressure, and is the void volume of the membrane, its membrane porosity, commonly around 80-85 %.
The size of pores does only enter indirectly into the equation; in their combination they comprise L, the thickness of the liquid layer, the membrane being some 80-85 % porous. The critical measurement of a flaw is the thickness of the liquid layer. Therefore a flaw or an oversized pore would be measured by the thinning of the liquid layer due to the elevated test pressure on the upstream side. The pore or defect may not be large enough that the Bubble Point comes into effect, but the test pressure thins the liquid layer enough to result into an increased gas flow. Therefore filter manufacturer specify the Diffusive Flow integrity test limits as maximum allowable diffusion value. The larger the flaw or a combination of flaws the higher the diffusive flow.

Pressure Hold Test
The pressure hold test, also known as pressure decay or pressure drop test, is a variant of the diffusive airflow test. The test set-up is arranged as in the diffusion test except that when the stipulated applied pressure is reached, the pressure source is valved off (Figure 9.). The decay of pressure within the holder is then observed as a function of time, by using a precision pressure gauge or pressure transducer.
The decrease in pressure can come from two sources, a.) the diffusive loss across the wetted filter. Since the upstream side pressure in the holder is constant, it decreases progressively all the while diffusion takes place through the wetted membrane or b.) the source of pressure decay could be a leak of the filter system setup.
An important influence on the measurement of the Pressure Hold test is the upstream air volume within the filter system. This volume has to be determined first to specify the maximum allowable pressure drop value. The larger the upstream volume, the lower the pressure drop or the smaller the upstream volume, the larger the pressure drop. This means also an increase in sensitivity of the test, but also an increase of temperature influences, if changes occur. Filter manufacturers specify maximum allowable pressure drop values.

Water Intrusion Test
The Water Intrusion test, also known as Water Flow Integrity test, is used for hydrophobic vent and air membrane filters only. The upstream side of the hydrophobic filter cartridge housing is flooded with water. The water will not flow through the hydrophobic membrane. Air or nitrogen gas pressure is then applied to the upstream side of the filter housing above the water level to a defined test pressure. This is done by way of an automatic integrity tester. A period of pressure stabilization takes place over, by the filter manufacturer recommended, timeframe, during which the cartridge pleats adjust their positions under imposed pressures. After the pressure drop thus occasioned stabilizes, the test time starts and any further pressure drop in the upstream pressurized gas volume, as measured by the automatic tester, signifies a beginning of water intrusion into the largest (hydrophobic) pores, water being incompressible. The automated integrity tester is sensitive enough to detect the pressure drop. This measured pressure drop is converted into a measured intrusion value, which is compared to a set intrusion limit, which has been correlated to the bacteria challenge test. As with the Diffusive Flow test, filter manufacturers specify a maximum allowable water intrusion value. Above this value a hydrophobic membrane filter is classified as non-integral.

Product Wet Integrity Testing
In cases of using the actual product as a wetting agent to perform integrity tests require the evaluation of product integrity test limits. The product can have an influence on the measured integrity test values due to surface tension or solubility. A lower surface tension for example would shift the Bubble Point value to a lower pressure and could result in a false negative test. The solubility of gas into the product could be reduced, which could result in false positive tests. Therefore a correlation of the product as wetting agent to the, by the filter manufacturer established, water wet values has to be done. This correlation is carried out by using a minimum of three filters of three filter lots. Depending on the product and its variability, one or three product lots are used to perform the correlation. The accuracy of such correlation is enhanced by automatic integrity test machines. These test machines measure with highest accuracy and sensitivity and do not rely on a human judgement as with a manual test. Multipoint diffusion testing offers the ability to test the filters performance, but especially the plot the entire diffusive flow graph through the Bubble Point. The individual graphs for a water wet integrity test can now be compared to the product wet test and a possible shift evaluated. Furthermore the multipoint diffusion test enables an improved statistical base to determine the product wet versus water wet limits.

Automated Integrity Testing
Most integrity tests are nowadays performed with qualified automated integrity test systems, which allow an easy and accurate test of choice by the end-user. The end-user programs the test parameter into the unit and when required chose the test needed for the specific filter, connect the unit to the pre-wetted filter system and let the machine run the test. The integrity test machine testing the filter system is by far more accurate than a manual test. Besides these machines provide a hardcopy print-out and/or have the ability to store the results in a data storage system.