MIC Calculator — Antibiotic Susceptibility

The MIC is read as the lowest antimicrobial concentration in a doubling dilution series that leaves a well free of visible growth. Because the series is geometric with a factor of two, results fall on a base-2 lattice of 0.25, 0.5, 1, 2, 4 mg/L and so on, and the reported value is interval censored: the true inhibitory concentration lies somewhere between the scoring well and the next lower one. This is why MIC data are usually analyzed as log2 values and why agreement within plus or minus one doubling dilution is the accepted standard for reproducibility between laboratories. Reference broth microdilution fixes the variables that shift the result, using a standardized inoculum near 5 x 10 to the fifth CFU/mL, cation-adjusted Mueller-Hinton broth, incubation at 35 degrees C for 16 to 20 hours, and growth and sterility controls on every plate. Interpretation requires breakpoints, which are organism and drug specific and are revised annually by CLSI and EUCAST from pharmacokinetic and pharmacodynamic modeling, wild-type distributions and clinical outcome data. An MIC alone does not predict success, since efficacy depends on exposure relative to the MIC: time above MIC for beta-lactams, and area under the curve divided by MIC for glycopeptides and fluoroquinolones. Known limitations include the inoculum effect, where a heavier inoculum raises apparent MICs for beta-lactams, trailing growth with azole antifungals, and heteroresistant subpopulations that a single endpoint read cannot detect.

Susceptibility testing began with observation rather than measurement. Alexander Fleming noted in 1929 that his Penicillium filtrate inhibited staphylococci at high dilution and later used a ditch plate to compare organisms, but tube dilution assays only became routine in the 1940s as penicillin and streptomycin entered use and clinicians needed to know whether an isolate would respond. Different laboratories used different media, inocula and endpoints, so the same strain could be called sensitive in one hospital and resistant in another. Two developments made the modern MIC possible. Gyula Takatsy, a Hungarian virologist, devised the microtitration plate and calibrated loops in the early 1950s, which turned twofold dilution series into a routine 96-well operation. Then came standardization: Alfred Bauer, William Kirby, John Sherris and Marvin Turck published their standardized disk diffusion procedure in 1966, Hans Ericsson and Sherris reported an international collaborative study on dilution methods in 1971, and the National Committee for Clinical Laboratory Standards, founded in 1968 and renamed the Clinical and Laboratory Standards Institute in 2005, began issuing the M07 dilution standard. EUCAST was established in 1997 under ESCMID and has since harmonized European breakpoints, publishing freely available tables that are revised each year.