What Does "Intermediate" Mean on Culture and Sensitivity (C&S)?
"Intermediate" (I) on a culture and sensitivity report indicates that the bacteria may be inhibited by the antibiotic at higher, achievable concentrations, but treatment success is uncertain and depends on achieving maximum dosing, favorable pharmacokinetics at the infection site, or the specific clinical context.
Definition and Clinical Significance
Intermediate susceptibility means the bacteria fall into a "gray zone" where the minimum inhibitory concentration (MIC) is at the upper limit of what the antibiotic can achieve in the body, making clinical response less predictable than with "susceptible" organisms 1.
The intermediate category reflects breakpoint MIC values that are arbitrarily defined based on achievable antibiotic levels in tissues, the distribution of MICs for that organism, and correlation with clinical outcomes 1.
Bacteria classified as intermediate have MIC values between the susceptible and resistant breakpoints, indicating the antibiotic may still work but with reduced reliability 1.
When Intermediate Results May Still Allow Treatment Success
Intermediate results may predict clinical success when the antibiotic concentrates well at the infection site (e.g., urinary tract infections where drugs achieve high urinary concentrations, or respiratory infections with antibiotics that concentrate in lung tissue) 1.
Using maximum recommended doses of the antibiotic can sometimes overcome intermediate susceptibility by achieving higher tissue concentrations 2.
The clinical context matters: an intermediate result for a superficial infection may be more concerning than for a urinary tract infection where drug concentrations are naturally higher 1.
Clinical Decision-Making with Intermediate Results
When an organism shows intermediate susceptibility, consider switching to an antibiotic showing full susceptibility whenever possible, as this provides more predictable clinical outcomes 3.
If no fully susceptible alternatives exist, treatment with the intermediate agent may be attempted using maximum dosing, particularly if the infection site allows good drug penetration 2.
Intermediate results should prompt discussion between the microbiologist and clinician to determine the best course of action based on infection severity, site, and available alternatives 3.
Common Pitfalls to Avoid
Do not assume intermediate means "partially effective" or "50% likely to work" - it specifically indicates uncertainty about achieving adequate drug levels to inhibit the organism 1.
Avoid using standard or reduced doses when treating intermediate organisms, as suboptimal dosing increases the risk of treatment failure and resistance development 3, 2.
Do not ignore intermediate results and continue empiric therapy without reassessment - this represents an opportunity to optimize treatment before clinical failure occurs 3.
Failing to consider the infection site when interpreting intermediate results can lead to either unnecessary antibiotic changes or continued use of suboptimal therapy 1.