What do S, I, and R mean?

S means susceptible at standard dosing, R means resistant, and I has different meanings by standard. Under EUCAST the I category means susceptible with increased exposure, such as higher or more frequent dosing, rather than borderline resistance. CLSI treats I as an intermediate buffer zone.

How is the category decided from the MIC?

An MIC at or below the susceptible breakpoint is reported S, an MIC above the resistant breakpoint is reported R, and an MIC between the two is reported I. When the susceptible and resistant breakpoints are the same value there is no intermediate band.

Are these breakpoints current and complete?

No. The values here are representative of published tables for a handful of common combinations and are meant for learning and quick orientation. Breakpoints are revised regularly, so always confirm against the live EUCAST or CLSI table for your exact organism, drug, and year.

Why do EUCAST and CLSI sometimes disagree?

The two bodies set breakpoints using different pharmacokinetic assumptions, dosing regimens, and clinical data, so the same MIC can be called S by one and I or R by the other. That is why the standard you select changes the result, and why labs report which standard they follow.

The minimum inhibitory concentration is the lowest antibiotic concentration that visibly stops an organism growing in vitro, measured on a doubling-dilution series such as 0.25, 0.5, 1, and 2 mg per litre. It is the quantitative input that breakpoints convert into a clinical category.

What is the Antibiotic MIC Breakpoint Interpreter (EUCAST/CLSI)?

Enter antibiotic, organism, and MIC to get the susceptibility category S, I, or R against representative EUCAST or CLSI breakpoints, with the susceptible and resistant cut-offs shown. For clinical microbiology and stewardship teams. It runs free in your browser on Gera Tools, with nothing uploaded.

Antibiotic MIC Breakpoint Interpreter (EUCAST/CLSI)

Name: Antibiotic MIC Breakpoint Interpreter (EUCAST/CLSI)
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

The minimum inhibitory concentration tells you how much antibiotic it takes to stop an organism growing, but a clinician needs a category, not a number. Breakpoint tables published by EUCAST and CLSI convert an MIC into susceptible, intermediate, or resistant, and this interpreter applies that rule for common combinations.

How it works

Each organism and drug pairing has two cut-offs, a susceptible breakpoint and a resistant breakpoint, both expressed in mg per litre. The classification follows a simple comparison: an MIC less than or equal to the susceptible breakpoint is reported S; an MIC greater than the resistant breakpoint is reported R; and an MIC that falls above the susceptible cut-off but at or below the resistant cut-off is reported I.

When the two breakpoints are the same value there is no intermediate band, so anything above the susceptible cut-off is reported resistant. MICs are measured on a doubling-dilution series, so a result sits on standardised values such as 0.25, 0.5, 1, 2, 4, and 8 mg per litre. Because the input is quantised this way, a borderline organism can move a whole category on a single dilution step — which is part of why the intermediate band exists at all, and why the standard you choose changes the call.

An illustrative example

Suppose, for example, a lab reports an MIC of 2 mg per litre for a given drug and organism, and the breakpoint table in use lists a susceptible cut-off of 1 mg per litre and a resistant cut-off of 4 mg per litre. Because 2 is greater than the susceptible cut-off of 1 but not greater than the resistant cut-off of 4, the result falls in the intermediate band and is reported I. The same MIC of 2 would be reported S if the susceptible cut-off were instead 2 or higher, and R if the resistant cut-off were instead below 2 — which is exactly how two standards with different cut-offs can disagree on an identical number. These figures are illustrative only; always read the actual cut-offs from the current official table for your organism, drug, and year.

EUCAST vs CLSI: why they differ

EUCAST (European Committee on Antimicrobial Susceptibility Testing) and CLSI (Clinical and Laboratory Standards Institute) both publish breakpoints based on pharmacokinetic/pharmacodynamic modelling and clinical outcome data, but they use different dosing regimens and datasets, which is why the same MIC can land in different categories depending on which standard the lab follows.

The most important conceptual difference is the meaning of the I category:

EUCAST I = susceptible, increased exposure. This was redefined in 2019. An I result signals that the drug is likely to work if dosing is maximised — higher dose, more frequent dosing, extended infusion, or an anatomical site where drug concentrations are naturally higher. It is not a warning of near-resistance.
CLSI I = intermediate. CLSI retains the traditional interpretation: I is a buffer zone that accommodates technical variability in the test. It implies uncertain clinical correlation and may or may not respond to standard or increased dosing.

This distinction is clinically significant. An EUCAST I result often supports treatment; a CLSI I result warrants caution.

Reading the tool output

The tool shows:

The category (S, I, or R) based on the standard you selected.
The susceptible breakpoint (≤) and resistant breakpoint (>) used to make the call.
A brief note on the clinical meaning of the category for the selected standard.

Important limitations

The breakpoints in this tool are representative examples drawn from published tables and are intended for education, orientation, and stewardship training. They are not a substitute for the current official document:

EUCAST publishes breakpoint tables at eucast.org, updated annually (usually January).
CLSI publishes M100 (Performance Standards for Antimicrobial Susceptibility Testing), updated yearly.

Full tables cover hundreds of species–drug combinations with detailed footnotes covering route of administration, infection site, dosing schedule, and resistance mechanism exceptions. Always consult the current edition for any clinical decision.