What does intermediate sensitivity mean in culture and sensitivity results and can the corresponding antibiotic be used?

Understanding Intermediate Sensitivity in Culture and Sensitivity Results

Antibiotics with intermediate sensitivity can be used in specific situations where the antibiotic can achieve higher concentrations at the infection site or when dosage can be increased, but should not be first-line choices when fully susceptible alternatives are available. 1

What Does Intermediate (I) Sensitivity Mean?

According to the European Society of Clinical Microbiology and Infectious Diseases (EUCAST) guidelines, intermediate sensitivity represents a classification for bacteria that fall between clinically susceptible and clinically resistant categories. This designation serves two important purposes:

1. It indicates that infections caused by these bacteria have variable or indeterminate responses to the antibiotic in question
2. It functions as a technical buffer zone for organisms with MICs (Minimum Inhibitory Concentrations) that are close to the breakpoint 1

When Can Intermediate Sensitivity Antibiotics Be Used?

Intermediate sensitivity antibiotics may still be effective in specific clinical scenarios:

• When the antibiotic can be concentrated at the infection site (e.g., urinary tract infections for certain antibiotics)
• When higher dosages of the antibiotic can be safely administered
• When limited treatment options are available 1

This is supported by FDA labeling for certain antibiotics like trimethoprim-sulfamethoxazole, which notes that for urinary tract infections specifically, "a report of 'Intermediate susceptibility' also indicates that the infection is likely to respond" 2.

Clinical Decision Algorithm for Using Intermediate Antibiotics

1. First priority: Use antibiotics reported as "Susceptible" whenever available

2. Second priority: Consider intermediate sensitivity antibiotics only when:

   • No susceptible options are available
   • The infection is localized to a site where the antibiotic concentrates (e.g., urinary tract)
   • Higher doses can be safely administered
   • The patient has previously responded to this antibiotic despite intermediate sensitivity

3. Third priority: Resort to resistant antibiotics only when no other options exist and after consultation with infectious disease specialists

Important Caveats and Pitfalls

• Infection site matters: The likelihood of treatment success with intermediate antibiotics varies by infection site. For example, intermediate sensitivity antibiotics are more likely to be effective for urinary tract infections than for bloodstream infections 2

• Dosing considerations: When using an intermediate antibiotic, consider using the maximum safe dose to overcome the reduced susceptibility

• Monitor closely: Patients treated with intermediate sensitivity antibiotics should be monitored more carefully for treatment failure

• Consider combination therapy: In severe infections, combining an intermediate antibiotic with another agent may improve efficacy

• Resistance emergence risk: Using antibiotics with intermediate sensitivity may increase the risk of selecting for fully resistant strains, especially with inadequate dosing or duration 3

Practical Application

When interpreting culture results:

• "S" (Susceptible): First choice for therapy
• "I" (Intermediate): Consider use only in specific situations as outlined above
• "R" (Resistant): Avoid use as "infection is unlikely to respond even to maximum doses" 1

Remember that intermediate sensitivity is a technical classification that serves as a buffer between susceptible and resistant categories, and clinical judgment must be applied when deciding whether to use these antibiotics in specific patient scenarios.