Constraints and feasibility of filter integrity testing for the production of sterile preparations in hospital pharmacy

Background
The production of hospital sterile preparations (HSP) requires the use of sterilising filters (1). At the end of production, a filter integrity test performed by measuring the bubble point (BP), for which technical specifications (filter wetting solvent, pressure limits) are rarely available or spartan. (1)
Objectives: The main objective of this work was to define the operating conditions and specifications of the BP tests performed on all the sterilising filters used in our HSP productions.

Materials and methods
The filter integrity test performed using a medical device to inject medical air into tubing connected to the filter under study (1). The BP tested on a pressure scale from zero to 3500 mbar. The obtained value allows to determine more accurately a common pressure range for testing the conformity of filters. The three types of sterilizing filters used in our HSP production include A-filters (cellulose, 4 cm², max. filtration: 100 mL), B-filters (polyethersulfone, 0.1 µm hydrophobic polyvinylidene fluoride vent filter, 10 cm², max. filtration: 2 L) and C-filters (dual-layer polyethersulfone, 300 cm²). There was no supplier data available for the BP of A-filters. The specifications for the BP test of C-filters included wetting the filter with isopropyl alcohol (IPA), while those of B-filters indicated wetting the filter with a 70° ethyl alcohol solution (EtOH 70).

Results and discussion
Before and after use, the BP values obtained for A-filters with EtOH 70 were higher (1500 mbar) than those obtained with IPA (1100 mbar). In contrast, for B-filters, the BP values obtained were variable (450 to 1650 mbar) or indeterminate. The variability would be attributed to the geometry of the filter with an air vent, incompatible with this type of test. Another technology of PB measuring by electron flow propagation could be a suitable solution for the control of B-filters (2). The BP results obtained from C-filters were homogeneous and were generally around 1600-1650 mbar.

Conclusion
This work has demonstrated the complexity of implementing reliable data collection regarding filter integrity testing, despite the importance of this step in controlling HSP production.

References
1. Meyer J. Filter testing: Parallel integrity testing of small venting filters. Filtration & Separation. nov 2009;46(6):22‑4.
2. Gormley GJ. Integrity testing: Testing filter membranes. Filtration & Separation. janv 2008;45:14‑7.

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