Statistical properties and inference of the antimicrobial MIC test

A common method for measuring the drug-specific minimum inhibitory concentration (MIC) of an antibacterial agent is via a two-fold broth dilution test known as the MIC test. Because this procedure implicitly rounds data upward, inference based on unadjusted measurements is biased and overestimates bacterial resistance to a drug. We detail this test procedure and its associated bias, which, in many cases, has an expected value of approximately 0.5 on the log(2) scale. In addition, new bias-corrected estimates of resistance are proposed. A numeric example is used to illustrate the extent to which the traditional resistance estimate can overestimate the true proportion of resistant strains, a phenomenon which is remedied by using the proposed estimates.