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.

Antibiotic MIC Breakpoint Interpreter (EUCAST/CLSI)

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:

S  if  MIC <= susceptible breakpoint
R  if  MIC >  resistant breakpoint
I  if  susceptible < MIC <= resistant

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 values such as 0.25, 0.5, 1, 2, and 4 mg per litre.

Reading the category and notes

The meaning of the middle category differs by standard. Under EUCAST, I now stands for “susceptible, increased exposure”, signalling that the drug can still work if dosing is intensified, rather than warning of near-resistance. CLSI retains I as an intermediate buffer that absorbs technical variability.

The breakpoints built into this tool are representative examples for orientation and teaching. They are revised every year, and the full tables cover hundreds of combinations with footnotes and exceptions. Always confirm against the current EUCAST or CLSI document for the exact organism, drug, and revision before making any clinical decision.