Antimicrobial susceptibility results tell you whether a specific antibiotic can effectively kill or stop the growth of the bacteria causing an infection. These results typically appear as a table listing antibiotics alongside a category: S (susceptible), I (intermediate), or R (resistant). Understanding what each category means, and what the numbers behind them represent, helps you make sense of the report and understand why your provider chose a particular treatment.
What S, I, and R Actually Mean
Every susceptibility report assigns each tested antibiotic one of three categories. S (Susceptible) means the bacteria are likely to respond to that antibiotic at standard doses. This is the green light: treatment with that drug has a high probability of success under normal dosing.
R (Resistant) means the bacteria are unlikely to respond to that antibiotic, even at higher doses. A resistant result is a clear signal to avoid that drug, because the organism has mechanisms to survive it.
I (Intermediate) is the most misunderstood category. The European Committee on Antimicrobial Susceptibility Testing (EUCAST) now formally defines it as “Susceptible, Increased Exposure.” That means the antibiotic can still work, but only if the drug reaches the infection site at higher-than-standard concentrations. This might happen through a higher dose, a longer infusion time, or simply because the infection is in a location where the drug naturally concentrates, like the urinary tract. An “I” result does not mean the antibiotic is useless. It means the treatment requires adjustment.
How the Lab Generates These Categories
Labs use two main methods to test susceptibility, and both produce a measurable number that gets translated into S, I, or R.
Disk diffusion involves placing small disks soaked in different antibiotics onto a plate of agar where bacteria are growing. If an antibiotic works, the bacteria can’t grow near the disk, creating a visible clear ring called a zone of inhibition. The lab measures the diameter of this zone in millimeters using a caliper. A larger zone means the bacteria are more sensitive to that drug. A standard 150-millimeter plate typically holds up to 12 antibiotic disks.
Minimum inhibitory concentration (MIC) testing takes a more precise approach. The lab exposes the bacteria to a series of wells containing increasing concentrations of an antibiotic. The MIC is the lowest concentration, measured in micrograms per milliliter, that completely prevents visible bacterial growth. A lower MIC means the bacteria are easier to kill with that drug.
Both the zone diameter and the MIC value are then compared against established cutoff numbers called breakpoints. If the zone is large enough or the MIC is low enough, the result is reported as susceptible. If the zone is too small or the MIC is too high, it’s resistant. Values in between fall into the intermediate category.
Who Sets the Breakpoints
Two major organizations establish the breakpoint values that labs use to interpret raw numbers into S, I, and R. In the United States, the Clinical and Laboratory Standards Institute (CLSI) publishes updated breakpoint tables annually. In Europe and increasingly worldwide, EUCAST sets the standards. Both organizations revise their breakpoints as new data emerges.
These revisions can meaningfully change results. When CLSI updated its breakpoints for minocycline against a drug-resistant bacterium called Acinetobacter baumannii in 2025, the susceptibility rate dropped from 73.9% under the old cutoffs to just 46.4% under the new ones. The bacteria hadn’t changed. The interpretation did. This is why the same organism tested in different years, or interpreted using different guidelines, can receive different categories.
Why MIC Values Matter More Than Categories Alone
If your report includes actual MIC numbers, they provide more useful information than the S/I/R label by itself. Two bacteria can both be classified as “susceptible” to the same antibiotic, but one might have an MIC of 0.5 and the other an MIC of 8. Both fall below the breakpoint, but the first organism is far more sensitive. If your provider has a choice between two antibiotics that both show “S,” the one with the lower MIC may offer a wider margin of safety.
The MIC also connects directly to how the antibiotic works in your body. For some drug classes, what matters most is how long the drug’s concentration in your blood stays above the MIC. Penicillins and related antibiotics (beta-lactams) work this way: they kill bacteria best when they maintain constant pressure above that threshold. For other drugs, like aminoglycosides, what matters is achieving a peak blood concentration that’s many times higher than the MIC. Fluoroquinolones depend on total drug exposure over 24 hours relative to the MIC. Your provider uses these relationships to select not just the right drug but the right dose and schedule.
When Lab Results Don’t Match Real-World Outcomes
A susceptibility test happens in a controlled lab environment: bacteria on a plate or in a well, exposed to a known concentration of antibiotic. Your body is far more complex, and several factors can cause a disconnect between the lab result and how well treatment actually works.
Drug levels vary dramatically by body site. An antibiotic might reach high concentrations in urine but barely penetrate bone, an abscess, or the fluid around the brain. A “susceptible” result assumes the drug can actually reach the bacteria at an adequate concentration, which isn’t always the case. This is one reason urinary tract infections can sometimes be treated with antibiotics that would be ineffective for bloodstream infections caused by the same organism: the drug concentrates in urine at levels far exceeding its MIC.
Your immune system also plays a role the lab can’t measure. A person with a weakened immune system may fail treatment even when the antibiotic should theoretically work. Conversely, someone with a robust immune response might recover despite borderline susceptibility, because the antibiotic only needs to weaken the bacteria enough for the immune system to finish the job.
Biofilms present another challenge. Some bacteria, particularly on medical devices like catheters or joint implants, form protective communities encased in a sticky matrix. Bacteria inside biofilms can tolerate antibiotic concentrations hundreds of times higher than their MIC measured in standard lab conditions. A “susceptible” result may be misleading when a biofilm is involved.
How Infection Location Shapes Interpretation
Where the infection is located changes which antibiotics make sense, even when the susceptibility report shows multiple options. Urinary tract infections illustrate this well. The most common culprit, E. coli, is followed by other organisms like Proteus mirabilis and Klebsiella pneumoniae. Because many antibiotics concentrate in urine at levels much higher than in blood, drugs that might show intermediate results for a bloodstream infection can work well for a bladder infection.
Complicated infections, those involving structural abnormalities, obstruction, immune suppression, or foreign bodies like catheters, tend to involve organisms with more resistance. The susceptibility report becomes even more critical in these cases because there are fewer effective options and less room for error.
What “Intermediate” Means in Practice
An intermediate result doesn’t automatically disqualify an antibiotic. In some clinical scenarios, providers intentionally use drugs in the intermediate category by increasing the dose. Research on tuberculosis treatment has demonstrated this approach: by using higher doses of first-line drugs for organisms in the intermediate range, clinicians can avoid switching to more toxic second-line medications. For example, using a higher dose of isoniazid expanded the treatable MIC range and preserved the use of a less toxic, more effective drug.
The key principle is that “intermediate” means the standard dose probably won’t generate enough drug exposure, but a modified approach might. Whether that modification makes sense depends on the drug’s safety profile at higher doses, the infection’s location, and whether better alternatives exist on the report.
Turnaround Times for Results
Traditional susceptibility testing requires growing bacteria from a sample and then exposing colonies to antibiotics, a process that takes 24 to 48 hours after the culture turns positive. Since the culture itself can take a day or two, the total wait from sample collection to susceptibility results is often two to four days.
Newer rapid testing systems can produce results in 2 to 7 hours from a positive blood culture, a significant improvement for serious infections like bloodstream infections where every hour on the wrong antibiotic matters. Some molecular tests can detect specific resistance genes directly from a sample without waiting for bacteria to grow at all. The tradeoff is that these rapid methods typically test fewer antibiotics and may not be available for all specimen types.
Reading a Susceptibility Report Step by Step
When you look at a susceptibility report, start by identifying the organism. The report will name the bacteria that grew from your culture. Next, scan the antibiotic list for “S” results, as these are the primary treatment options. If MIC values are included, lower numbers within the susceptible range suggest stronger activity.
Look at the “R” results to understand which drugs won’t work. If most antibiotics show resistance, this signals a harder-to-treat organism, and your provider may need to use a less common drug or combination therapy.
Pay attention to which antibiotics were tested. Labs select a panel based on the organism and infection type, so not every antibiotic will appear. The absence of a drug from the report doesn’t mean it wouldn’t work. It means it wasn’t tested, often because it’s not a first-line choice for that type of infection. If you’re curious about a specific antibiotic, your provider can request additional testing.