Understanding Culture Sensitivity Report Categories
When reading a culture sensitivity report, "sensitive" means the antibiotic will likely work at standard doses, "intermediate" means the infection may respond only with higher doses or at sites where the drug concentrates, and "resistant" means the infection is highly unlikely to respond even at maximum doses. 1
Sensitive (S)
Bacteria classified as sensitive will be inhibited or killed by achievable concentrations of the antibiotic at standard dosing regimens. 1 This designation indicates:
- The organism lacks resistance mechanisms against this antibiotic 1
- Standard therapeutic doses should successfully treat the infection 1
- This is your preferred choice when available, as it represents the most predictable clinical response 1
Important caveat: Even bacteria reported as "resistant" on culture may respond clinically if the patient is already improving on empiric therapy—the infection's clinical response matters more than the lab result alone. 1
Intermediate (I)
Organisms with intermediate susceptibility fall into a gray zone between sensitive and resistant, where clinical response is variable or indeterminate. 1 This category means:
- The infection may be eliminated if the antibiotic concentrates at the infection site (such as in urine for urinary tract infections) or if you increase the dosage 1
- Standard doses may fail, but higher doses or specific anatomic concentration can achieve success 1
- This designation also serves as a technical buffer zone to account for organisms with MICs (minimum inhibitory concentrations) close to the breakpoint, minimizing laboratory classification errors 1
Clinical approach: Consider intermediate results as potentially treatable but requiring either dose optimization or selection of an alternative agent, particularly for severe infections where treatment failure carries high risk. 1
Resistant (R)
Resistant organisms possess resistance mechanisms that make infection highly unlikely to respond even to maximum antibiotic doses. 1 This indicates:
- The bacteria have phenotypic or genotypic resistance mechanisms 1
- Clinical failure is expected even with maximal dosing 1
- You should select an alternative antibiotic to which the organism is sensitive 1
Critical exception: If the patient is already clinically improving on empiric therapy despite reported resistance, you may continue the current regimen rather than switching—clinical response trumps in vitro data. 1 However, if the infection is worsening despite reported susceptibility, consider whether surgical intervention is needed, fastidious organisms were missed, or drug absorption/levels are inadequate. 1
Practical Application
When culture results return, reassess your empiric therapy using this algorithm: 1
- Patient improving + organism sensitive: Continue current therapy or narrow spectrum (de-escalation) 1
- Patient improving + organism resistant: Continue if clinically responding well; the resistant organism may be a colonizer rather than pathogen 1
- Patient not improving + organism sensitive: Consider surgical intervention, inadequate drug levels, or missed pathogens 1
- Patient not improving + organism resistant: Switch to an antibiotic covering all isolated organisms 1
Common pitfall: Pseudomonas aeruginosa is frequently isolated but often represents colonization rather than true infection—patients commonly improve despite therapy lacking anti-pseudomonal coverage. 1 Only target Pseudomonas if the patient has risk factors (high local prevalence, foot soaking, prior antibiotic failure, severe infection) or is not responding to initial therapy. 1
Breakpoints matter: The specific MIC values defining these categories are based on achievable antibiotic concentrations at the infection site, the distribution of MICs for that organism, and correlation with clinical outcomes. 1 These are standardized values but can vary between different testing standards. 1