How to choose an antibiotic based on a susceptibility report?

How to Choose Antibiotics Based on a Susceptibility Report

The most effective approach to selecting antibiotics based on a susceptibility report is to choose the narrowest-spectrum agent to which the organism is susceptible, considering patient-specific factors and the site of infection. 1

Understanding the Susceptibility Report

A susceptibility report typically provides:

• Identification of the pathogen
• List of antibiotics tested
• Results categorized as:
  • Susceptible (S): Pathogen likely to be inhibited by standard dosing
  • Intermediate (I): Higher doses may be needed or drug concentrates at infection site
  • Resistant (R): Pathogen unlikely to respond to the antibiotic

Step-by-Step Selection Algorithm

1. Identify the Pathogen

• Note the specific organism isolated (e.g., Staphylococcus aureus, Pseudomonas aeruginosa, Enterobacterales)
• Consider the site of infection (respiratory, urinary, bloodstream, etc.)

2. Evaluate Susceptibility Results

• Focus on antibiotics marked as "Susceptible" (S)
• Disregard antibiotics marked as "Resistant" (R)
• Consider "Intermediate" (I) only when limited options are available

3. Apply the Cascade Approach

• Start with narrow-spectrum agents first
• Progress to broader-spectrum only when necessary
• For example, with Staphylococcus aureus:
  • If MSSA: Choose oxacillin/nafcillin/cefazolin first 1
  • If MRSA: Choose vancomycin or linezolid 1

4. Consider Site-Specific Factors

• Respiratory infections: Select agents with good lung penetration 1, 2
• Urinary tract infections: Consider agents concentrated in urine 1
• Bloodstream infections: Choose bactericidal agents 1
• Bone/joint infections: Select agents with good bone penetration 1

5. Consider Patient-Specific Factors

• Allergies
• Renal/hepatic function
• Drug interactions
• Pregnancy status
• Age

Pathogen-Specific Recommendations

Gram-Positive Organisms

Staphylococcus aureus

• MSSA: Prefer oxacillin, nafcillin, or cefazolin 1
• MRSA: Vancomycin (15-20 mg/kg IV q8-12h) or linezolid (600 mg PO/IV q12h) 1

Streptococcus pneumoniae

• Penicillin-susceptible (MIC <2): Penicillin G, amoxicillin, or amoxicillin/clavulanate 1
• Penicillin-resistant (MIC ≥2): Based on susceptibility - ceftriaxone, fluoroquinolones, or vancomycin 1

Enterococcus species

• E. faecalis: Ampicillin, piperacillin-tazobactam, or vancomycin 1
• VRE: Linezolid or daptomycin 1

Gram-Negative Organisms

Pseudomonas aeruginosa

• Susceptible to multiple agents: Choose monotherapy with the narrowest spectrum agent 1
• Carbapenem-resistant: Ceftolozane/tazobactam, ceftazidime/avibactam, or colistin 1
• In septic shock: Consider combination therapy with two agents 1
• Avoid: Aminoglycoside monotherapy (except for UTIs) 1

Enterobacterales (E. coli, Klebsiella, etc.)

• ESBL-negative: Cephalosporins or fluoroquinolones based on susceptibility 1
• ESBL-positive: Carbapenems or piperacillin-tazobactam 1
• Carbapenem-resistant: Ceftazidime/avibactam, meropenem/vaborbactam, or polymyxin-based combinations 1

Acinetobacter species

• Carbapenem-susceptible: Carbapenem or ampicillin/sulbactam 1
• Carbapenem-resistant: Intravenous polymyxin with adjunctive inhaled colistin 1
• Avoid: Tigecycline 1

Common Pitfalls to Avoid

1. Automatically selecting the broadest-spectrum agent: This promotes antimicrobial resistance
2. Ignoring local resistance patterns: Consider hospital/regional antibiograms
3. Overlooking bioavailability: Some antibiotics have excellent oral bioavailability (fluoroquinolones, linezolid) while others require IV administration for serious infections
4. Misinterpreting "Intermediate" results: These may be effective with higher dosing or at sites where the drug concentrates
5. Failing to de-escalate: Once susceptibility results are available, narrow therapy to the most targeted agent 1
6. Not considering penetration to infection site: Some antibiotics penetrate poorly into certain tissues (e.g., CNS, bone)

Special Considerations

• Biofilm-associated infections (e.g., prosthetic joints): Consider antibiotics with biofilm activity like fluoroquinolones or rifampin combinations 1
• Polymicrobial infections: May require combination therapy to cover all pathogens 1
• Immunocompromised patients: May need broader initial coverage 2
• Severe infections: Consider higher doses or extended/continuous infusions for time-dependent antibiotics 1

By following this systematic approach, you can optimize antibiotic selection based on susceptibility reports, improving patient outcomes while minimizing the development of antimicrobial resistance.