Are multidose over-the-counter artificial tears adequately preserved?
OBJECTIVE
To investigate artificial tears containing different preservatives for antimicrobial efficacy. Based on the challenge test outlined in the European Pharmacopoeia, products were tested in their original containers to see whether their component preservative had sufficient activity.


METHODS
Five brands of over-the-counter artificial tears each containing different preservatives (benzalkonium chloride/EDTA, parabens, chlorobutanol, silver chloride complex, and Purite-stabilized oxychloro complex) were inoculated with test microorganisms (Pseudomonas aeruginosa, Staphylococcus aureus, and Candida albicans). Changes in the microbial start concentration with time were followed by plating onto growth media to provide a measure of the preservative efficacy. In another test, artificial tears were applied to paper disks that were then placed onto agar growth media seeded with microorganisms. Zones of inhibition were measured after incubation.


RESULTS
Only the brand of artificial tears containing benzalkonium chloride/EDTA satisfied the major criteria for antimicrobial preservation for all the test microorganisms. Only a benzalkonium chloride/EDTA-containing disk placed on agar seeded with S. aureus produced a zone of inhibition in the agar diffusion test.


CONCLUSIONS
The brand of artificial tears containing benzalkonium chloride/EDTA is suitable for sale in countries adopting the monographs of the European Pharmacopoeia. Other brands would only be suitable for sale if justified reasons for not meeting the major criteria for preservative efficacy can be provided.
Charnock
Colin
C
Department of Health Sciences, Oslo University College, Oslo, Norway. colin.charnock@hf.hio.no