MICvärden

MICvärden, or minimum inhibitory concentration values, indicate the lowest concentration of an antimicrobial agent that prevents visible growth of a microorganism after standardized incubation. They are used to assess the in vitro susceptibility of pathogens and aid in selecting effective therapy. MIC values are expressed in micrograms per milliliter (µg/mL) or milligrams per liter (mg/L).

Testing is performed by standardized methods such as broth microdilution, broth macrodilution, agar dilution, or gradient diffusion (Etest). Inocula are prepared to specified densities, and incubation is typically at 35°C for 16–20 hours, though conditions vary by organism and method. The result is the MIC, the lowest antimicrobial concentration that inhibits growth.

Interpretation uses breakpoints set by guidelines such as CLSI or EUCAST, which classify isolates as susceptible, intermediate (or susceptible-dose dependent), or resistant. Breakpoints depend on organism, antibiotic, and dosing; PK/PD data influence these thresholds. Clinicians consider the MIC alongside achievable drug concentrations and tissue penetration.

MIC values are also summarized across populations using MIC50 and MIC90, representing the concentrations inhibiting 50% and 90% of isolates, respectively, and are used for surveillance and resistance trend assessment.

Limitations include that in vitro MICs do not guarantee clinical outcomes, and results can be affected by inoculum size, media, pH, growth phase, and presence of biofilms or heteroresistance. Quality control with reference strains is essential to ensure accuracy.