We perform many eye (conjunctiva) cultures on elderly nursing home patients. Most of our clients treat infected eyes with topical antimicrobial agents basing their decision on our reported MIC result. Is there a correlation between MIC values and topical antimicrobials? Is there a more appropriate method for testing the effectiveness of topical antimicrobials? 

This is a frequently asked question. There are no standard recommendations for interpreting MIC results for agents that are used topically. This was addressed by NCCLS in 1997 in M7-A4 (disk diffusion testing). On page 20 in the "Question and Answer" section, the following comment appeared: "The interpretive criteria outlined in this document are intended for systemically administered antimicrobial agents. To date, the subcommittee has not had the opportunity to review relevant studies that correlate in vitro susceptibility testing results with clinical response data for topical agents. Until specific guidelines can be developed, clinically significant isolates from eye infections for which interpretive criteria exist may be tested and reported in an effort to develop clinical correlations."