What I and R Mean in Culture Reports
In culture and antimicrobial susceptibility testing, "I" stands for Intermediate susceptibility and "R" stands for Resistant, representing standardized clinical categories that guide antibiotic selection based on the likelihood of treatment success.
Understanding the Three-Category System
Culture reports categorize bacterial susceptibility into three clinical categories based on breakpoint values:
- Susceptible (S): The infection is highly likely to respond to standard antibiotic dosing 1
- Intermediate (I): The infection has variable or indeterminate response to therapy, but may be eliminated if the antibiotic is concentrated at the infection site or if dosage is increased 1
- Resistant (R): The infection is highly unlikely to respond even to maximum doses of the antibiotic 1
What "Intermediate (I)" Means Clinically
A bacterium classified as intermediate lies between clinically susceptible and clinically resistant strains. 1
Key clinical implications:
- Infections caused by intermediate strains may still respond to treatment if higher doses are used or if the drug concentrates at the infection site 1
- The intermediate category provides a technical buffer zone that minimizes confusion for organisms with MICs close to the breakpoint 1
- Treatment success is variable and depends on achieving adequate drug concentrations at the site of infection 1
What "Resistant (R)" Means Clinically
Clinical resistance indicates that infection is highly unlikely to respond even to maximum doses of the antibiotic. 1
Two types of resistance exist:
- Microbiological resistance: The organism possesses resistance mechanisms demonstrated phenotypically or genotypically, which may be qualified as low-level, moderate, or high-level 1
- Clinical resistance: The practical inability to achieve therapeutic success even with maximum dosing, though other factors affecting therapy response can make this determination difficult 1
How These Categories Are Determined
The categorization is based on breakpoints—specific values of parameters such as MIC (Minimum Inhibitory Concentration) or inhibition zone diameters that assign bacteria to clinical categories 1
- MIC represents the lowest concentration (in mg/L) that prevents bacterial growth under defined conditions 1
- Reports contain both quantitative MIC results and qualitative interpretations (S, I, or R) 1
- Breakpoints are established by either CLSI (Clinical and Laboratory Standards Institute) in the USA or EUCAST (European Committee for Antimicrobial Susceptibility Testing) in Europe 1
Critical Clinical Considerations
When you see "I" or "R" on a culture report, antimicrobial susceptibility testing should always be performed to guide therapy when a microorganism is identified. 1
Important caveats:
- Environmental conditions at the infection site (oxygen tension, pH, protein binding) can affect actual clinical outcomes even when in vitro testing suggests susceptibility 2
- For serious infections, organisms reported as "I" may require dose optimization or alternative agents 2
- Organisms reported as "R" should prompt selection of alternative antibiotics to which the organism is susceptible 1
- Previous antimicrobial therapy is one of the most important risk factors for resistant pathogens 1