MIC testing has been standardised and refined over decades, and two major organisations now define the breakpoints used to interpret results: EUCAST (the European Committee on Antimicrobial Susceptibility Testing) and CLSI (the Clinical and Laboratory Standards Institute, based in the USA). The existence of two organisations with slightly different breakpoints for the same organism-antibiotic combinations creates real-world confusion, and understanding why the differences exist, and when they matter, is an important part of practical microbiology.
This page explains what MIC means and how it is measured, describes the reference method and the clinical alternatives, covers the EUCAST and CLSI breakpoint systems in plain language, connects MIC to pharmacokinetics and pharmacodynamics so you understand what the numbers mean in a patient, and walks through the situations where a sensitive-looking MIC still predicts treatment failure.
What MIC Actually Measures
MIC is determined by the broth microdilution method, the internationally recognised reference standard. A standard inoculum of bacteria (typically 5 x 10^5 colony-forming units per mL, prepared by comparing to a 0.5 McFarland turbidity standard) is added to a series of wells, each containing a two-fold dilution series of the antibiotic being tested (for example: 64, 32, 16, 8, 4, 2, 1, 0.5, 0.25, 0.125 mg/L). After 16 to 20 hours of incubation at 35 degrees Celsius, each well is examined for turbidity (visible growth). The MIC is the concentration in the lowest well where no growth is visible.
This is not the same as the lowest concentration that kills the bacteria. MIC measures inhibition of growth, not killing. The concentration that kills 99.9 per cent of bacteria is called the minimum bactericidal concentration (MBC), which is typically two to four dilutions higher than the MIC for bactericidal drugs. For most clinical purposes, MIC is the relevant measurement, because inhibiting bacterial growth is often sufficient in a patient with a functional immune system. For life-threatening infections in immunocompromised patients, achieving bactericidal activity may be more important.
From Broth to Bedside: The Three Testing Methods
Broth microdilution is the reference method. It is performed in microtitre plates, usually 96-well format, allowing multiple antibiotics and concentrations to be tested simultaneously. Automated systems like the Vitek 2 (bioMérieux), Phoenix (BD), and Sensititre (Thermo Fisher) use miniaturised versions of this principle with fluorescent or colorimetric growth indicators. These systems produce MIC values within 6 to 18 hours and are the backbone of most clinical microbiology laboratories worldwide.
Disc diffusion (the Kirby-Bauer method) is not an MIC method, but it is the most widely used susceptibility testing method globally because of its simplicity and low cost. Antibiotic-impregnated discs are placed on an agar plate inoculated with the test organism. The antibiotic diffuses outward creating a gradient, and where the concentration falls below the MIC, growth resumes. The diameter of the zone of inhibition around the disc is measured and compared to breakpoint zone diameters. EUCAST and CLSI each publish zone diameter breakpoints that correlate with MIC breakpoints, allowing disc diffusion results to be interpreted as susceptible, intermediate, or resistant.
Etest (gradient diffusion) combines the convenience of disc diffusion with direct MIC readout. An Etest strip contains a stable, predefined gradient of antibiotic along its length, ranging from very high to very low concentration. When placed on an inoculated agar plate, a teardrop-shaped zone of inhibition forms. Where the zone edge intersects the strip, the MIC value is read directly from a scale printed on the strip. Etest is particularly useful for slow-growing or fastidious organisms that are difficult to test by automated methods, and for confirmatory testing of isolates with unusual or borderline results.
EUCAST and CLSI: Two Breakpoint Systems, One Global Problem
Both EUCAST and CLSI publish breakpoints annually, based on large datasets combining microbiological, pharmacokinetic, and pharmacodynamic data. The goal of both is the same: to define cut-offs that identify organisms that will respond to standard doses of the antibiotic in clinical infections. The difference is in their methodology and the data sets they use.
EUCAST uses pharmacokinetic/pharmacodynamic (PK/PD) breakpoints derived from Monte Carlo simulations modelling drug concentrations in human tissues against MIC distributions for real-world bacterial populations. The result is breakpoints that reflect what happens to the drug in a human body at standard doses, in the tissues where infections occur. EUCAST also uses a three-category system: susceptible (S), susceptible only at increased exposure (I, formerly "intermediate"), and resistant (R). The EUCAST "I" category specifically means the drug is likely to work if the dose is increased or the infection is at a site where the drug concentrates.
CLSI breakpoints are also PK/PD-based but use slightly different source data and methodologies, leading to differences for some organism-antibiotic combinations. Laboratories need to clearly state which guideline they are using when reporting results, and clinicians need to understand that a "sensitive" result from a EUCAST lab may have a different numeric breakpoint than the same result from a CLSI lab.
MIC and Pharmacodynamics: Why the Number Only Makes Sense in a Patient
An MIC value on its own is not enough to predict whether a drug will work. What matters is whether adequate drug concentrations are achieved at the site of infection, for a sufficient duration or at a sufficient peak, relative to the MIC.
Beta-lactam antibiotics (penicillins, cephalosporins, carbapenems) are time-dependent killers. Their efficacy depends on the proportion of the dosing interval during which free drug concentration remains above the MIC at the infection site. For most beta-lactams, achieving fT greater than MIC of 40 to 50 per cent of the dosing interval predicts treatment success. This is why prolonged infusion of beta-lactams (infusing over 3 to 4 hours rather than 30 minutes) is increasingly used for treating infections caused by organisms with higher MICs, even when the isolate is technically still classified as susceptible.
Fluoroquinolones and aminoglycosides are concentration-dependent. Their efficacy correlates with the ratio of the peak drug concentration (Cmax) to the MIC (Cmax/MIC) or the total drug exposure over 24 hours relative to the MIC (AUC24/MIC). This is why aminoglycosides are often given as a single daily dose rather than multiple smaller doses: a high peak concentration relative to the MIC drives bacterial killing more effectively than sustained moderate concentrations.
Glycopeptides like vancomycin show efficacy that correlates with the AUC/MIC ratio. The target AUC0-24/MIC ratio for vancomycin against S. aureus is 400 to 600 mg/h/L. This is why vancomycin dosing has shifted away from trough-level monitoring toward AUC-guided therapeutic drug monitoring (TDM), particularly for serious MRSA infections. An organism with a vancomycin MIC of 1 mg/L requires more intensive dosing than one with an MIC of 0.5 mg/L, even though both are technically classified as susceptible under EUCAST (breakpoint S is equal to or less than 2 mg/L).
When Susceptible MIC Results Still Mean Treatment Failure
Understanding the clinical situations where the laboratory says "susceptible" and the patient fails treatment is one of the most important skills in applied microbiology.
Vancomycin MIC creep in MRSA is a well-documented phenomenon. As vancomycin MICs among S. aureus isolates have crept upward from 0.5 to 1 mg/L or to 1 to 2 mg/L over time (still technically susceptible), clinical failure rates for MRSA bacteraemia treated with vancomycin have increased. Many centres now switch to daptomycin or ceftaroline for MRSA bacteraemia when the vancomycin MIC is 1 mg/L or above.
Biofilm infections create a different problem. Bacteria embedded within biofilm on prosthetic devices, cardiac valves, or bone are hundreds of times more tolerant of antibiotics than their planktonic (free-floating) counterparts, even when MIC testing of the planktonic population shows susceptibility. The MIC, measured against a planktonic culture, does not predict activity against the biofilm-embedded population.
Intracellular infections present a related problem. The MIC measured in broth reflects drug activity in a liquid medium, not inside a macrophage or within a cell-rich tissue where an organism like Salmonella, Mycobacterium, or Legionella is actually living. Drugs with excellent MIC results may achieve inadequate intracellular concentrations.
Frequently Asked Questions
What does MIC stand for?
MIC stands for minimum inhibitory concentration. It is the lowest concentration of an antibiotic that prevents visible bacterial growth in a standardised broth dilution test.
What is the difference between MIC and MBC?
MIC is the lowest concentration that stops visible growth. MBC (minimum bactericidal concentration) is the lowest concentration that kills 99.9 per cent of bacteria. For bacteriostatic drugs, the MBC is much higher than the MIC. For bactericidal drugs, MBC is usually within one to two dilutions of the MIC.
How is MIC measured in a clinical lab?
Most clinical labs use automated broth microdilution systems (Vitek 2, BD Phoenix, Sensititre) that test multiple antibiotic concentrations simultaneously and report MIC values with automatic interpretation. Etest strips are used as a supplement or alternative. Disc diffusion gives zone diameters which are interpreted using published breakpoints, not a direct MIC value.
What is the difference between EUCAST and CLSI?
EUCAST is the European standard, and CLSI is the US standard. Both define MIC breakpoints for interpreting susceptibility results, but they use different methodologies and datasets, sometimes resulting in different breakpoint values for the same organism and antibiotic. Laboratories must clearly state which guideline they are using.
What does susceptible, intermediate, and resistant mean?
Susceptible (S) means the organism is likely to respond to standard doses of the drug. Resistant (R) means standard doses are unlikely to work. In the EUCAST system, I means susceptible only at increased exposure: the drug may work if the dose is increased or the infection is at a site where the drug concentrates well.
What is the clinical significance of vancomycin MIC creep?
Vancomycin MIC creep refers to the gradual increase in vancomycin MIC values among S. aureus isolates over time. Even when the MIC is still within the susceptible range (up to 2 mg/L by EUCAST), higher MICs within the susceptible range are associated with higher failure rates in MRSA bacteraemia. Many clinicians now prefer alternative agents like daptomycin for MRSA bacteraemia when the vancomycin MIC is 1 mg/L or above.
What is PK/PD and why does it matter for MIC?
Pharmacokinetics (PK) describes how the body handles a drug: absorption, distribution, metabolism, and elimination. Pharmacodynamics (PD) describes how the drug affects bacteria. PK/PD integration links the drug concentration profile in a patient's body (PK) to the MIC of the infecting organism (PD) to predict whether the drug will work in clinical practice. The same MIC can be treatable with one dosing regimen and untreatable with another.
What is time above MIC?
Time above MIC is a pharmacodynamic parameter used for time-dependent antibiotics like beta-lactams. It describes what proportion of the time between doses the free drug concentration at the infection site remains above the MIC. For most beta-lactams, maintaining free drug concentrations above the MIC for 40 to 70 per cent of the dosing interval is necessary for effective treatment.
What is MRSA and why is the MIC important for treatment?
MRSA (methicillin-resistant Staphylococcus aureus) carries the mecA gene, which encodes an altered penicillin-binding protein (PBP2a) with very low affinity for beta-lactam antibiotics. All beta-lactams are therefore ineffective regardless of the in vitro MIC. Treatment requires non-beta-lactam alternatives, most commonly vancomycin, daptomycin, or linezolid. The MIC of these agents against the specific MRSA isolate guides treatment decisions.
Can an organism be susceptible on a disc diffusion test but resistant by MIC?
Very rarely, technical differences between test methods can produce discordant results. If a clinical outcome does not match the susceptibility result, the isolate should be retested and ideally confirmed by MIC testing. More commonly, an organism may be correctly classified as susceptible but still cause treatment failure due to pharmacokinetic/pharmacodynamic factors, biofilm, or intracellular location.