Minimum Inhibitory Concentration (MIC) Calculator

The Minimum Inhibitory Concentration (MIC) Calculator determines the lowest concentration of an antimicrobial agent that prevents visible bacterial growth — a critical measurement in microbiology and antibiotic susceptibility testing. Enter your starting concentration, dilution factor, number of dilution wells, and the MIC well number where growth is first inhibited, along with positive control OD₆₀₀ and growth inhibition threshold. The calculator returns the MIC value in µg/mL, the lowest and highest concentrations in your dilution series, and an MIC interpretation.

µg/mL

Initial concentration of antimicrobial agent

wells

The well number where growth is first inhibited

Optical density of growth control well

%

Percentage inhibition to define MIC

Results

MIC Value

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Lowest Concentration in Series

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Highest Concentration in Series

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MIC Interpretation

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Results Table

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Frequently Asked Questions

What is the Minimum Inhibitory Concentration (MIC)?

MIC is the lowest concentration of an antimicrobial agent that inhibits visible growth of a microorganism. It's a key parameter in determining antibiotic effectiveness and resistance patterns.

How do I determine which well number represents the MIC?

The MIC well is the first well in the dilution series where growth is completely inhibited (typically >90% inhibition compared to the positive control). Count wells from highest to lowest concentration.

What dilution factor should I use for my assay?

Two-fold dilutions are most common for MIC testing as they provide good resolution while maintaining manageable concentration ranges. Use smaller dilution factors for more precise MIC determination.

Why is the positive control OD₆₀₀ important?

The positive control validates that the organism can grow under test conditions. An OD₆₀₀ of 0.5-1.0 indicates healthy growth and ensures your MIC results are reliable.

How do I interpret MIC values for clinical decision making?

Compare your MIC value to established breakpoints (susceptible, intermediate, resistant) for the specific organism-antibiotic combination. These breakpoints are published by organizations like CLSI or EUCAST.

What factors can affect MIC determination accuracy?

Key factors include inoculum size, incubation conditions, medium composition, and reading time. Standardized protocols like CLSI M07 help ensure reproducible results.

Can this calculator be used for antifungal susceptibility testing?

Yes, the same principles apply to antifungal MIC testing, though specific protocols and breakpoints may differ. Ensure you follow appropriate guidelines for fungal testing.

What should I do if I get inconsistent MIC results?

Check your inoculum preparation, ensure proper mixing of dilutions, verify incubation conditions, and consider testing multiple replicates. Quality control strains can help validate your methodology.