How do you interpret an antibiogram (antimicrobial susceptibility report)?

How to Read an Antibiogram

An antibiogram is a tool that summarizes bacterial susceptibility patterns to different antibiotics, classifying them as susceptible, intermediate, or resistant, which is essential for guiding appropriate empiric antibiotic therapy. 1

Understanding the Basic Components

Susceptibility Categories

• Susceptible (S): Indicates that the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches concentrations usually achievable 2
• Intermediate (I): Result should be considered equivocal; implies possible clinical applicability in body sites where the drug is physiologically concentrated 2
• Resistant (R): Indicates that the pathogen is not likely to be inhibited even at maximum antibiotic doses; alternative therapy should be selected 3, 2

Numerical Values and Interpretations

• MIC (Minimum Inhibitory Concentration): The lowest concentration of an antibiotic that prevents bacterial growth under defined laboratory conditions 3

  • Reported in μg/mL
  • Lower MIC values indicate greater susceptibility
  • Example interpretation for most organisms:
    • ≤1 μg/mL: Susceptible
    • 2 μg/mL: Intermediate
    • ≥4 μg/mL: Resistant 2

• Zone Diameter: For disk diffusion testing, measured in millimeters (mm)

  • Larger zones indicate greater susceptibility
  • Example for Enterobacteriaceae:
    • ≥21 mm: Susceptible
    • 16-20 mm: Intermediate
    • ≤15 mm: Resistant 2

How to Use an Antibiogram Effectively

Step 1: Identify the Relevant Section

• Locate the organism of interest (rows) and the antibiotics tested (columns)
• Note that susceptibility patterns vary significantly between different bacterial species and even within species

Step 2: Interpret Susceptibility Percentages

• Percentages represent the proportion of isolates susceptible to each antibiotic
• Higher percentages (>90%) indicate better empiric coverage
• Example: If E. coli shows 95% susceptibility to ceftriaxone but only 75% to ciprofloxacin, ceftriaxone would be preferred for empiric therapy

Step 3: Consider Stratified Data When Available

• Hospital-wide antibiograms may mask important susceptibility differences across units 3, 4
• Look for antibiograms stratified by:
  • Hospital unit (ICU vs. non-ICU) 4
  • Specimen type (blood, urine, respiratory) 4
  • Patient population (pediatric vs. adult) 3
  • Community vs. healthcare-acquired infections 5

Step 4: Note Temporal Trends

• Compare with previous years' antibiograms to identify emerging resistance patterns
• Decreasing susceptibility rates over time should prompt reassessment of empiric therapy choices

Important Considerations and Pitfalls

Common Pitfalls

1. Overlooking local resistance patterns: Hospital-wide antibiograms may overestimate or underestimate resistance in specific units 4, 5

   • Example: E. coli resistance to ciprofloxacin can range from 64.5% to 95.1% across different hospital departments 4

2. Ignoring patient risk factors: Susceptibility varies based on:

   • Previous antibiotic exposure (major risk factor for resistance) 3
   • Healthcare facility residence (significantly affects susceptibility) 5
   • Recent hospitalization 5

3. Misapplying data to atypical infections: Standard antibiograms may not reflect susceptibility in:

   • Mixed infections 3
   • Unusual infection sites where antibiotic concentrations differ 1

4. Failing to recognize limitations: Antibiograms:

   • Cannot account for exceptions or unforeseen mutations 3
   • Are typically updated annually, so may not reflect very recent changes in resistance patterns 6

Best Practices

1. Use the most specific antibiogram available for your patient population and infection site 3

2. Consider patient-specific risk factors that may increase likelihood of resistant organisms 5

3. Remember that antibiograms guide empiric therapy but should be adjusted once culture and susceptibility results are available 3

4. For critically ill patients, consider broader initial coverage with subsequent de-escalation based on culture results

5. Use antibiograms in conjunction with clinical guidelines for specific infection types to optimize therapy selection

By understanding how to properly interpret antibiograms, clinicians can make more informed decisions about empiric antibiotic therapy, improving patient outcomes while minimizing unnecessary broad-spectrum antibiotic use and development of resistance.