What is the difference between sensitivity and susceptibility to antibiotics in treating bacterial infections?

Sensitivity vs Susceptibility to Antibiotics in Treating Bacterial Infections

In antimicrobial therapy, "susceptibility" and "sensitivity" are synonymous terms that both refer to whether a bacterial infection will likely respond to a particular antibiotic at achievable drug concentrations, while "resistance" indicates that the infection is unlikely to respond even to maximum antibiotic doses. 1

Key Definitions and Distinctions

Susceptibility/Sensitivity

• Clinical Definition: A bacterial infection is classified as "susceptible" or "sensitive" when it is likely to respond to treatment with a particular antibiotic at standard dosages 1
• Microbiological Definition: Refers to bacteria that lack resistance mechanisms when tested in laboratory settings 1
• Practical Application: When a laboratory report indicates an organism is "susceptible" to an antibiotic, it suggests that standard doses of that antibiotic are likely to be effective in treating the infection 2

Resistance

• Clinical Resistance: Occurs when an infection is highly unlikely to respond even to maximum doses of a given antibiotic 1
• Microbiological Resistance: Refers to bacteria possessing any resistance mechanism that can be demonstrated either phenotypically or genotypically 1
• Levels of Resistance: Can be qualified as "low-level," "moderate," or "high-level" resistance 1

Intermediate Category

• Represents bacterial strains that fall between susceptible and resistant classifications
• Infections with intermediate susceptibility may respond to therapy if:
  • The antibiotic concentrates at the infection site
  • Higher dosages are administered 1

Measurement and Interpretation

Minimum Inhibitory Concentration (MIC)

• Defined as the lowest concentration of an antibiotic that prevents bacterial growth under defined laboratory conditions 1
• Serves as the primary laboratory measurement for determining susceptibility

Breakpoints

• These are specific MIC values or zone diameter measurements that define the categories of susceptible, intermediate, and resistant 1
• Breakpoints are established based on:
  • Pharmacokinetic properties of the antibiotic
  • Achievable drug concentrations at infection sites
  • Clinical response data

Antibiograms

• Profiles showing the susceptibility pattern of bacteria to different antibiotics
• May be expressed as a resistance profile (e.g., PST) or as a series of S (susceptible), I (intermediate), or R (resistant) designations 2

Clinical Implications

Susceptibility Testing in Practice

• All successful antimicrobial therapy should be susceptibility-based to achieve high cure rates (≥95%) 1
• Therapy selection can be guided by:
  • Direct susceptibility testing of the patient's isolate
  • Local population data showing which empiric therapies remain effective 1

Avoiding Treatment Failure

• Using antibiotics against resistant organisms leads to treatment failure and contributes to the spread of resistance 3
• Infections caused by resistant bacteria lead to up to two-fold higher rates of adverse outcomes compared to infections with susceptible strains 3

Resistance Development

• Resistance often emerges not through de novo evolution during treatment but through selection of already resistant strains 4
• Patient history of prior antibiotic exposure is crucial in predicting potential resistance 1

Common Pitfalls to Avoid

• Misinterpreting laboratory reports: Remember that "sensitive" and "susceptible" mean the same thing in clinical microbiology reports
• Ignoring local resistance patterns: Local antibiograms should guide empiric therapy choices
• Failing to confirm treatment success: Post-treatment testing is essential to verify cure and monitor for emerging resistance 1
• Overlooking site-specific considerations: Antibiotic concentrations vary by tissue, affecting clinical efficacy even when laboratory tests show susceptibility 2

Best Practices for Antimicrobial Stewardship

• Obtain thorough history of prior antibiotic use before selecting therapy 1
• Consider susceptibility testing before initiating treatment for serious infections 1
• Use the shortest effective duration of therapy to minimize selection pressure for resistance 5
• Confirm treatment success with appropriate follow-up testing 1
• Share local resistance data to guide community prescribing practices 1