MIC50

MIC50 is the minimum inhibitory concentration at which 50% of the tested microbial isolates are inhibited in vitro under standardized laboratory conditions. It is a statistical descriptor derived from the distribution of MIC values obtained for a given antimicrobial agent against a defined population of organisms.

Calculation and interpretation: to determine MIC50, MICs are obtained for a panel of isolates, then ordered from lowest to highest. The MIC value at the 50th percentile (the median) is reported as the MIC50. If the number of isolates is even, the mean of the two central values may be used. MIC50 provides a central tendency of potency across the tested population and is often reported alongside MIC90, which denotes the concentration inhibiting 90% of isolates.

Usage: MIC50 is commonly used in antimicrobial surveillance and research to compare the relative potency of antibiotics, to monitor shifts in susceptibility over time (MIC creep), and to summarize large data sets. It complements other descriptors such as MIC90, MIC range, and distribution curves. It is not a clinical breakpoint.

Relation to breakpoints: clinical breakpoints (susceptible, intermediate, resistant) are established by organizations like CLSI and EUCAST and depend on pharmacokinetic/pharmacodynamic data, dosing, infection site, and outcomes. MIC50 itself does not determine susceptibility categories but helps illustrate overall potency and distribution within a population.

Limitations: MIC50 does not convey the full range or shape of the MIC distribution, can be influenced by sample size and selection, and MIC values are typically in twofold dilution steps, which can affect precision. It is most informative when presented with MIC90 and distribution data for context.