Fifteen hydrophobic cartridge vent filters are integrity tested using diffusion and intrusion (water breakthrough) methodologies. Several filters are deliberately punctured. All are then re-tested to determine the ability of each method to detect the loss in sterilizing capability. Bacterial challenge per HIMA guidelines of selective filters is performed as a confirmation of sterilizing integrity. Data obtained do indicate an effectiveness of both the diffusion and the water intrusion test method to identify losses in integrity. However, water intrusion or breakthrough results show a limitation in ability to consistently identify those filters with compromised sterilizing capability. Inconsistent fill volumes are addressed as a source of error in breakthrough testing. Inherent advantages and disadvantages of the two methodologies are presented. Though the convenience of breakthrough testing is offset by an inability to identify a loss in membrane integrity, there remain circumstances where breakthrough testing may have a place.