Understanding ">2" on Urine Culture Sensitivity Reports
When you see ">2" on a urine culture sensitivity report, this indicates the minimum inhibitory concentration (MIC) is greater than 2 μg/mL, meaning the bacteria require more than 2 μg/mL of that antibiotic to inhibit their growth. 1
What MIC Represents
MIC is the lowest concentration of an antibiotic (expressed in mg/L or μg/mL) that prevents visible bacterial growth under standardized laboratory conditions. 1
The true inhibitory concentration lies between the reported MIC value and the next higher concentration tested in the dilution series. 2
Lower MIC values indicate greater bacterial susceptibility—the antibiotic is more effective at lower concentrations. 2
Interpreting ">2 μg/mL"
The ">" symbol means the actual MIC exceeds the highest concentration tested, which in this case was 2 μg/mL. 1
This notation indicates the laboratory did not test higher concentrations, so the exact MIC remains unknown but is definitively above 2 μg/mL. 1
For context, CDC defines decreased azithromycin susceptibility as ≥2.0 μg/mL for certain organisms, illustrating that ">2" often signals reduced susceptibility or resistance depending on the antibiotic and organism. 1
Clinical Significance
You must compare this MIC value to established clinical breakpoints for that specific organism-antibiotic combination to determine if the bacteria are susceptible, intermediate, or resistant. 3
Susceptible (S) means MIC is at or below the breakpoint; Intermediate (I) falls between susceptible and resistant thresholds; Resistant (R) exceeds the breakpoint and likely results in clinical failure. 3
For example, with ciprofloxacin against most aerobic organisms, the susceptibility breakpoint is ≤1 μg/mL, intermediate is 2 μg/mL, and resistant is ≥4 μg/mL. 4 Therefore, ">2" would indicate at least intermediate resistance, possibly full resistance.
Common Pitfall to Avoid
Never assume ">2" means the antibiotic will work clinically—this value may exceed the resistance breakpoint for many organism-drug combinations. 3
Environmental conditions at the infection site (oxygen tension, pH, protein binding) can dramatically affect antibiotic activity beyond what MIC predicts. 2, 3
Always check your laboratory's specific breakpoint interpretations, as these vary by organism and testing methodology. 4
Practical Application for Urinary Tract Infections
For UTIs, antibiotics with high urinary concentrations can sometimes achieve success despite higher MICs, as drug levels in urine far exceed serum levels. 3
However, if the MIC is in the intermediate range or the organism shows resistance, select an alternative antibiotic with a lower MIC value and documented susceptibility. 3
Tigecycline susceptibility is defined as MIC = 2 μg/mL for certain resistant organisms, demonstrating that context matters when interpreting specific MIC values. 1