Filter integrity test method

Filter integrity testing methods are classified in two types, 

1) Destructive test

2) Non destructive test

The goal of filter integrity test is to ensure product quality, product performance and reduce the risk.

Filter or filtration membrane are made up of PTFE (polytetrafluoroethylene) or PVDF (polyvinylidene fluoride), PTFE is more hydrophobic than PVDF.

Destructive test is generally performed by manufacturer based on statistical sample from each lot of membrane and fabricated devices produced.

Non destructive testing perform by manufacturer on each sterilizing grade filter prior to sale to ensure it's integrity. (Ref. Merk Millipore. Com "Integrity Testing methods".)

The product deliver to end user are integrity tested but once the product exit from manufacturing company there will be a chance of damage during shipment/supply to end user so it's preferred to perform pre and post use integrity by the end user to ensure product integrity.

1) Destructive filter integrity test:-

Bacterial challenge test is used as destructive test. In this test solution of culture medium containing bacteria introducing at Upstream of filter and ensuring zero organisms are recovered post filter.

This test is called destructive because in this test intentionally microbial culture is loaded and after it can't be use longer.

It cannot return to the service and must be destroyed.

Millipore bacterial retention test 0.22 µm filter discs and devices are challenged with a solution of culture medium containing bacteria (Brevundimonas diminuta ATCC 19146) at a minimum challenge of 10⁷  per cm2. The effluent is then passed through a second 0.45 µm assay filter disc that is placed on an agar plate and incubated to check the growth, if no growth observed that means filter intact and if growth observed than filter fail the test. 

2) Non destructive test:-

This test is performed at user end before and after uses of filter.

There are three types of non destructive tests,

i) Bubble point test

ii) Diffusion test

Variation of diffusion tests are 

a) Pressure hold

b) Forward flow

c) Pressure decay

iii) Water intrusion test or Water flow test

(Ref. Merk Millipore. Com "Integrity Testing methods".)

i) Bubble point test:-

The Bubble point is the amount of pressure that, when applied to the Upstream side of the filter, overcomes the water tension of the largest pore in the system and forces the water through the pore where air is now freely flowing through that pore. (Ref. ICQconsultant .com "know the difference between three main filter integrity test".)

This test is most popular for small filtration devices. It is used for both hydrophobic (Water repulsive) and hydrophilic (Water attractive) filter.

This test can be performed manually or with an automated device that measure the minimum Bubble point value.

This test is based on the fact that liquid is held in the pores of the filter by surface tension and capillary forces.

It is used to measure the pore diameter by measuring the minimum pressure required to force liquid out of the pores.

Bubble point expressed as,

BP= 4*k*γ.cosѲ/d

K= shape correction factor,

γ = Surface tension

Ѳ = Contact angle

d = Pore diameter

Procedure:-

- Wet the filter with appropriate fluid, (water for hydrophilic filter and alcohol/water mixture for hydrophobic filter.

- Pressurize the system to 80% of the expected Bubble point Pressure from manufacturing literature.

- Slowly increase the pressure until rapid continues bubbling is observed at outlet.

- The point where the Bubble stream observed is the Bubble point of that filter.

Manual Testing:-

-This test can be performed manually same as automated in which pressure gauge is used in test housing for pressure check. 

-The 80% of test pressure shall be applied at the Upstream side of filter manually and allow to stabilize.

-The outlet is deep in the water so continues bubbling can be observed.

-Slowly increased the pressure until the Bubble stream is observed at outlet.

-The point where the Bubble stream observed is the Bubble point of that filter.

Test result measured in term of pressure (Bubble point in mbar).

Minimum bubble point is provided by manufacturer and same has to be used during test program development while maximum bubble point is user dependent to set during test development.

(e.g. For the filter if manufacturer provide the minimum bubble point 1300 mbar, so this is the filter passing criteria)

Diffusion test:-

The diffusion test applies a constant pressure of air on the upstream side of a filter saturated with liquid and measure the volume of air over time that diffuses through the filter. 

At differential gas pressures, gas molecules migrate through the water filled pores of a wetted membrane following fick's law of diffusion.

The gas diffusional flow rate is proportional to the differential pressure (upstream and downstream side of the filter) and the total surface area of the filter.

The diffusion test pressure is lower than the minimum bubble point as gas dissolved in the liquid held in pores and start to diffuse out.

This test can be performed manually or with an automated device.

Test Procedure:-

- Wet the filter with appropriate fluid, (water for hydrophilic and alcohol/water mixture for hydrophobic filter.

- Slowly increase pressure on the upstream side of the filter to the recommended test pressure provided by the manufacturer.

- Allow the system to equilibrate.

- In automated device gas diffusion measured by device while for manual need to measure the gas flow at the outlet for one minute with an inverted graduated cylinder or a flow meter.

Test result measured in term of volume of diffusion in ml.

Pressure hold test/ Pressure decay/ Pressure drop test:- 

This test is the variation  of the diffusion test. In this test a highly accurate gauge is used to monitor upstream pressure changes due to gas diffusion through the filter. The pressure hold value is dependent on the diffusional flow and upstream volume. It can be calculated using the following equation,

Pressure hold (Δp) = D *T* Pa/ Vh

Δp = Pressure drop (bar or psi),
D = Diffusion rate (ml/min),
T = Time (minute),
Pa = Atmosphere pressure ( 1 atm or 14.7 psi),
Vh = Upstream volume of apparatus (ml).

Water intrusion test:-

This test is used for hydrophobic filters. In this test filter is fixed in housing and filled with ambient temperature water (WFI). Allow to stabilize so air bubble can be removed and water make a layer over the membrane surface. Test pressure is applied at the upstream side of filter, and measure the pressure drop due to water vapor passing through the membrane. (Ref. Best practices for successful filter integrity testing using water intrusion test method, Biopharma asia.com)

Advantages: 

-This test is becoming more popular as it removes the need for a solvent based wetting.
-Since the membrane does not wet out during the WIT, the filter can be used after draining and a minimal drying period reducing filter downtimes.

- It can be performed on filters in sterilized system, as there is no manipulation in downstream side of the filter.

 Factor affecting to filter integrity tests:-

1) Temperature:- The temperature play major role in integrity test so the temperature of area, test gas (air or nitrogen), filter temperature, filter housing temperature, wetting agent or test liquid temperature should be ambient during test. 

Since WIT relies on evaporative flow of water across the membrane to be tested any increase in temperature will result in a high evaporation rate leading to higher water flow measured.  

The gas used for test should be at ambient temperature so that it is neither heating up nor cooling down during the test which causes falsification in result.

Using the ideal gas law as a rule of thumb, homogeneous temperature change of 1°C inside the sample during the diffusion measurement phase can affect the diffusion test result by about 25%. (Ref. ISPE.org PHARMACEUTICAL ENGINEERING, risk assessment for thermal influences on filter and container closure integrity testing, by Magnus Andreas Stering).

2) Test gas:- Test gas should be clean, dry and at ambient temperature.

When performing a diffusion test, the type of test gas has to be considered. The maximum allowable diffusion value for a cartridge is typically given for air as the test gas if no longer gas is stated. As nitrogen (N2) has a lower solubility in water than air, the diffusion rate for a given filter will be reduced. If nitrogen is being used instead of compressed air, the maximum allowable diffusion rate must be modified, (Diffusion_MaxN2  = Diffusion_maxAir  * 0.82).

3) Wetting agent/ Test fluid:- The same wetting agent shall be used as mentioned by manufacturer in product specification. In WIT any material that can change the surface tension of water affect the test result (e.g. Any contaminating materials, natural oil, water conductivity as evaporation of water is higher at lower conductivity), in Bubble point test and Diffusion test different wetting agent gives false result due to change the surface tension. Filter wetting procedure and wetting time also play major role in test result.

                   

4) Test Program:- There are different test program for different filter, test program parameters shall be set as per manufacturer recommendation.

5) Filter housing:- Filter housing shall be filter specific, housing design and housing volume affect the result.