A critical evaluation of the Multi-item Microbial Challenge Test in ophthalmic disinfectant testing

The Multi-item Microbial Challenge Test (MIMCT) is a stringent, carrier assay of the disinfecting efficacy of a contact lens care regimen. The test involves four steps: 1) in vitro contamination of contact lenses in the presence of organic load; 2) cleansing the contact lens by mechanical rubbing and subsequent rinsing; 3) disinfection of the cleansed lens; and 4) assaying the contact lens and the disinfecting solution for viable microorganisms. The variability of the individual steps was evaluated in this study by the Contributions of Elements protocol. The efficacy of the cleansing step, while reputed to be highly variable, was found to be reproducible and dependent upon the formulation under test. A contact lens disinfecting regimen must reduce the level of contaminants to approximately 10(-4) CFU/lens to enjoy a 95% chance of passing the MIMCT. This reflects a 10-log unit reduction from the initial inoculum of approximately 10(6) CFU/lens. The stringency of the MIMCT depends on the ability of the recovery system to allow the growth of low numbers of residual challenge microorganisms in the presence of the disinfecting solution. Neutralizers included in the recovery system to inhibit the disinfectant are therefore critically important to the stringency of the assay. The design of experiments to evaluate neutralizers specifically for the MIMCT is discussed with suggestions for an improved procedure.