Sterile filter cartridges meet below standard: Challenge this filter with defective pseudomonas of 107CFU per square centimeter, and downstream liquids are sterile.
2 Sterile Filter Classification and Integrity Testing Method
Usually there are two kinds of sterile filters, hydrophilic and hydrophobic. Different integrity testing methods are applied to the two kinds of filters with different character. Bubble point and diffusion for hydrophilic filters, water intrusion testing and bubble point for hydrophobic filters. Location where a filter is set and used, function and filtrate will be considered thoroughly to choose the suitable method.
Below is the figure for integrity testing of a filter:
3 Common reasons of integrity testing failure
Several common reasons of integrity testing failure as below:
3.1 Membrane wetting: Full membrane wetting is necessary before integrity testing, make sure enough flush volume, flow rate, pressure, time and wetting liquid temperature.
3.2 Residual filtrate among membrane: Some filtrate will be left among membrane after filtration. Residual filtrate’s surface tension is different from wetting liquid’s, affecting contact angle. Then testing will fail, which is common.
3.3 Testing environment: Testing temperature will affect testing result.
3.4 Incomplete system: two main cases: (1) The filter is damaged. (during sterilization much large differential pressure, differential pressure causing by different temperature-fall rates of upstream and downstream during cooling process after sterilization, broken membrane due to frequent pressure pulse during filtration, broken spear, loose seal or broken seal O-Ring); (2) Leakage (usually happen upstream: leakage of system connection, leakage of integrity test instrument’s vent outlet).
3.5 Incorrect system setting: connectors don’t put well (connected valve doesn’t insert into chuck). Valve inside testing instrument don’t open, downstream valve isn’t open.
3.6 Breathable filter of collection tank is full of water.
4 Initial analysis and solution after fail testing
If the testing result is failed, first, we need to make an analysis and troubleshoot it, then choose proper measurements to solve it.
4.1 First check the system setting, refer to below figure:
4.2 For broken filters or leakage, usually flow rate is large. Pressure maintaining could be taken. Check the leakage point firstly. Disinfectants can be sprayed at clamps to find leak points. Tighten it after finding. If no leakage is found, the probable reason is that the filter is damaged or setting of filter is not sealed well.
4.3 If the previous pressure curve is normal, it’s marginal failure. Bubble point value or flow volume is a little lower than standard. It’s due to incomplete wetting or residual filtrate. Just flush the filter thoroughly. Or confirm the integrity test data on the basis of filtrate, use filtrate for testing, no need to flush the filter by water.
4.4 If bubble point value is too high to be close to bleed pressure, or flow rate is too low to be close to zero, testing result may be not true. The valve connecting to integrity test instrument don’t set well or isn’t open. Examine testing system, find out and solve the problem, repeat the testing.
4.5 Water intrusion test may be failed as the filter is wet because of sterilization. Then test again after drying the filter.