An antibiogram is a chart that shows which antibiotics work against which bacteria. It comes in two forms: a report for an individual patient showing the results of a specific culture, and a facility-wide cumulative antibiogram that summarizes resistance patterns across an entire hospital or clinic. Both use the same core language, and once you understand the key components, reading either one is straightforward.
The Two Types of Antibiograms
An individual antibiogram is the susceptibility report attached to a patient’s culture results. The lab grows the bacteria from a sample (urine, blood, wound swab), identifies the organism, and then tests it against a panel of antibiotics. The report tells clinicians exactly which drugs will kill that specific strain.
A cumulative antibiogram is a hospital-wide summary, usually updated once a year. It lists the most commonly encountered bacteria down the left side and antibiotics across the top, with each cell showing the percentage of tested isolates that were susceptible. If a cell reads “85%” for a given bug-drug combination, that means 85% of the strains tested at that facility were killed by that antibiotic. Clinicians use cumulative antibiograms to choose empiric therapy, meaning the best-guess antibiotic to start before an individual patient’s culture results come back. A general threshold: if susceptibility falls below 80%, many stewardship programs recommend choosing a different empiric option.
What S, I, and R Mean
Every antibiogram classifies each bug-drug combination into one of three categories:
- S (Susceptible): The bacteria are killed or stopped from growing at normal doses of the antibiotic. This is the green light.
- I (Intermediate): The bacteria survive at low concentrations of the drug but not at higher ones. Treatment may still work, but only if the antibiotic naturally concentrates at the site of infection (as some drugs do in urine, for example) or if higher doses are used.
- R (Resistant): The bacteria are not killed by the antibiotic at achievable doses. This drug should not be used.
For certain drug-bug combinations, you may also see SDD, which stands for Susceptible-Dose Dependent. This replaced the intermediate category in select cases and means the antibiotic can still work, but only with higher doses, more frequent dosing, or extended infusion times. When you see SDD on a report, the standard dose will not be enough.
Understanding MIC Values
Some antibiogram reports go beyond S, I, and R and include a number called the MIC, or minimum inhibitory concentration. This is the lowest concentration of the antibiotic, measured in micrograms per milliliter, that completely stops the bacteria from growing in a lab dish. A lower MIC means the bacteria are easier to kill; a higher MIC means they are harder to kill.
The MIC number alone does not tell you much. It has to be compared against published breakpoints, which are threshold values set by standardization bodies. If the MIC falls at or below the susceptible breakpoint, the organism gets an “S.” If it falls above the resistant breakpoint, it gets an “R.” Values in between land in the intermediate or SDD range. The two main organizations that set these breakpoints are CLSI (used primarily in the United States) and EUCAST (used primarily in Europe). Their breakpoints sometimes differ for the same drug-bug combination, which is why two labs in different countries might categorize the same MIC differently.
Breakpoints are not permanent. They get updated periodically as new dosing data, resistance trends, and clinical outcomes emerge. A strain that was classified as susceptible five years ago could be reclassified as intermediate today if the breakpoint changed.
How to Read a Cumulative Antibiogram Table
A typical cumulative antibiogram is structured as a grid. Bacteria are listed in rows, often grouped into categories like Gram-positive (Staphylococcus, Enterococcus, Streptococcus) and Gram-negative (E. coli, Klebsiella, Pseudomonas). Antibiotics fill the columns, sometimes grouped by drug class. Each cell contains a percentage representing the proportion of isolates susceptible to that drug.
To use the table, find the organism you are interested in, then scan across the row to see which antibiotics have the highest susceptibility percentages. A cell showing 95% is a strong empiric choice. A cell showing 40% means more than half of recent strains at that facility were resistant. Some cells will be blank, which usually means the lab did not test that particular combination, either because the drug is not relevant for that organism or because too few isolates were available to generate a reliable percentage.
Keep in mind that cumulative antibiograms reflect the facility’s overall patient population. If you are dealing with a patient who has had multiple recent courses of antibiotics or recurrent infections, their individual resistance profile may be worse than what the cumulative data suggests.
Spotting Key Resistance Patterns
Certain resistance patterns have names you will see referenced alongside antibiogram data. Recognizing them helps you interpret results more quickly.
MRSA (methicillin-resistant Staphylococcus aureus) shows up as resistance to oxacillin or cefoxitin on the antibiogram. When a Staph aureus isolate is resistant to these drugs, it is resistant to nearly all standard penicillins and cephalosporins, regardless of what individual results for those drugs might suggest. The report may flag this explicitly.
ESBL-producing bacteria (most commonly E. coli and Klebsiella) carry enzymes that break down many penicillins and cephalosporins. Most labs do not directly test for ESBL production. Instead, resistance to ceftriaxone (specifically, an MIC of 2 micrograms per milliliter or higher) is often used as a proxy to suspect ESBL production, though this is not perfectly specific.
VRE (vancomycin-resistant Enterococcus) is identified by resistance to vancomycin on the antibiogram. This is clinically significant because vancomycin is normally a go-to drug for serious Enterococcus infections.
Why “Susceptible” Does Not Always Mean “Best Choice”
When multiple antibiotics come back as susceptible on a report, the right pick is not simply whichever one appears first. Several factors matter beyond the S on the page.
The site of infection is critical. An antibiotic that works well in a test tube might not reach adequate concentrations in certain body compartments. Some drugs concentrate heavily in urine, making them ideal for urinary tract infections but inadequate for bloodstream or deep tissue infections caused by the same organism. Current guidelines advise choosing the narrowest-spectrum antibiotic that targets the most likely pathogen. Using a broad-spectrum drug when a narrow one would work contributes to resistance and increases the risk of complications like C. difficile infection.
Side effect profiles, drug allergies, kidney or liver function, and cost all factor in. A susceptible result opens the door, but it does not make the decision by itself.
Intrinsic Resistance: When Results Can Mislead
Some bacteria are naturally resistant to certain antibiotics due to their biology, not because they acquired resistance. These are called intrinsic resistances, and a well-designed antibiogram should not even test or report these combinations. But it helps to know the major ones so you are not confused by their absence from a report.
Gram-negative bacteria, for example, are intrinsically resistant to macrolide antibiotics like erythromycin. Their outer cell membrane and efflux pumps prevent the drug from accumulating inside the cell. You will never see erythromycin listed as an option against E. coli on an antibiogram, and that is by design. Similarly, all E. coli strains carry a chromosomally encoded enzyme that provides a baseline level of resistance to certain penicillins, which is why those drugs are not tested against it in standard panels.
If you see an antibiotic missing from a report, it often means the lab already knows that combination is clinically irrelevant, either because of intrinsic resistance or because the drug is not appropriate for the type of infection being treated.