Resistance is where the MIC of a drug is above the recommended breakpoint concentration and so is likely to result in treatment failure.

Antibiotic resistance allows bacteria to survive in the presence of an antibiotic and is typically measured in vitro. However, this may not be clinically relevant. This is because the level of drug resistance (as measured by the minimum inhibitory concentration, MIC) needs to be higher than the levels of the clinically achievable levels of the drug at the site of infection. Recommended breakpoint concentrations are a guide as to whether the MIC indicates clinically relevant drug resistance. Clinically relevant antibiotic resistance has a significant impact on patient outcomes, leading to longer hospital stays, a need for second-line or last-resort antibiotics, increased likelihood of needing intensive care, and increased mortality.

Specific genes are associated with antibiotic resistance. Clinically relevant antibiotic resistance genes can be found in bacteria residing in the human microbiome as well as in bacteria in the environment, and animals. These can act as reservoirs that facilitate the acquisition of antibiotic resistance genes by bacteria that can infect humans.