The effect of interlaboratory variability on antimicrobial susceptibility determination

In the minimum inhibitory concentration (MIC) test literature, discussion concerning the effect of laboratory-to-laboratory variation is lacking. We present 2 sets of drug dilution test quality control data that illustrate considerable laboratory differences in measured MIC. In both isolates (Escherichia coli, ATCC 25922; Staphylococcus aureus, ATCC 29213) the laboratory-to-laboratory variability accounts for approximately half of the total variability. We illustrate the impact of this variability on the probability of correctly classifying the susceptibility level of an isolate and on the estimation of resistance prevalence. For example, we show that laboratory differences in the probability of correctly classifying the isolate (specifically near the lower breakpoint) can vary up to 80%.