Control of microbial contamination in unpreserved eyedrops

Preservatives are added to multidose eyedrop containers to ensure that the eyedrops do not become heavily contaminated during patient use. This enables eyedrops to be used for up to 1 month after opening. However, patients are frequently required to use unpreserved eyedrops as their eyes are unable to tolerate these preservatives. Some commercial unpreserved unit dose eyedrops are available, but the range is limited, and they cannot be used in all cases. Twenty one different unpreserved eyedrop formulations in multidose bottles were tested to establish their inherent efficacy in antimicrobial preservation, and to help determine a suitable in use storage life.

The eyedrops were inoculated with a known quantity of four different micro-organisms according to the method of the European Pharmacopoeia 'Test for the efficacy of antimicrobial preservation'. After set periods of time, samples were taken to determine the number of viable organisms remaining.

Antibiotics and alkaloids were generally shown to have higher kill rates than other eyedrops such as artificial tears and steroids.

In general, once opened by individual patients in a domiciliary situation, a 7 day in use storage life is confirmed for eyedrops containing alkaloids or antibiotics, if they are stored in the refrigerator after opening.