The European Pharmacopeia test for efficacy of antimicrobial preservation was mainly designed for evaluating liquid dosage forms: eye drops, nose drops, liquids for external use, liquids for oral administration, parenteral solutions. The loading test consists of artificially contaminating the preparation by means of representative types of micro-organisms and monitoring their regression over the course of time. This test, which implies the sampling of homogeneous aliquots over the course of time, is not suitable for dosage forms which are solid or semi-solid at room temperature.

A test method is proposed for zinc gelatin from the Swiss Phannacopoeia 8, which could be adapted to other nonfluid preparations. A splitting of the preparation is proposed to overcome the problem that the preparation is solid at ambient temperature. In such a case, another change was also implemented consisting of replacing the sample at "time zero" by a neutralised control, in order to take into account the bactericidal effect of the zinc and the physical state of the preparation. Furthermore, in the case of Pseudomonas aeruginosa and Candida albicans, it was suggested that their count on a solid medium be complemented by an enrichment method in a liquid medium, which enables the detection of bacteria which are damaged but nevertheless still viable and which makes it possible to lower the limits of detection when checking whether or not a preparation meets the acceptance criteria.

In view of the overall results, it would appear that the zinc gelatin preparations not containing any preservative (DAB JO or USP XXI) or containing 0.1 % m/m of methyl parahydroxybenzoate (MPHB) as a preservative (Ph. Helv. 8 or DAB 7) do not meet the acceptance criteria of the European Pharmacopoeia test for efficacy of antimicrobial preservation.