Introduction
The rational use of antimicrobials is a key element in developing a successful strategy against the development of resistance to antimicrobials. The physician should establish the need and the reason for therapy, select the appropriate antimicrobial agent, and then decide on the optimum dose and dosing interval, duration, as well as route of administration.
Reasons for appropriate antibiotic use:
- To cure or prevent infections using the best agent available in the shortest time and with the least side effects.
- To minimise the emergence and spread of resistant microorganisms.
- To provide healthcare services at a reasonable cost.
General Principles in Choosing an Antimicrobial Agent
- Choose drugs based on known or suspected pathogen and susceptibility patterns.
- Opt for the narrowest effective spectrum.
- Consider drug side effects, interactions, and patient-specific factors (e.g., pregnancy, renal function).
- Cost is relevant after efficacy and safety.
- Consider likelihood of patient compliance.
Key Considerations in Prescribing
Representative Specimen Collection
- Collect specimens before starting therapy.
- Avoid contamination.
- Cultures guide definitive therapy—early antibiotics can sterilise cultures.
Empiric Therapy
- Base on probable pathogens and local resistance patterns.
- Awareness of common pathogens in specific clinical settings is key.
Adjusting Therapy Based on Lab Results
- Use susceptibility testing: disc diffusion, MIC, breakpoint testing, or E-tests.
- Helps determine resistance and guide targeted treatment.
Monitoring and Measuring
Therapeutic Response
- Clinical response often sufficient.
- Non-response may indicate resistance, incorrect dosing/route, superinfection, or need for surgery.
Drug Level Monitoring
- For drugs like aminoglycosides and vancomycin:
- Avoid toxicity, especially in renal impairment.
- Ensure adequate levels during long-term therapy.
Pharmacokinetics & Pharmacodynamics
- Pharmacokinetics: how the body affects the drug.
- Pharmacodynamics: how the drug affects the body.
Principles:
- Drug must bind target (e.g., ribosomes, PBPs).
- Concentration and time at infection site are crucial.
- MIC = minimum concentration needed to inhibit bacterial growth.
Antibiotic Types
Time-Dependent Antibiotics
- Penicillins, cephalosporins, carbapenems, macrolides.
- Effectiveness depends on time above MIC.
- Frequent dosing or prolonged infusion may be required.
Concentration-Dependent Antibiotics
- Quinolones, aminoglycosides.
- Killing depends on high peak/MIC ratios (≥10).
- Once-daily dosing often preferred.
Duration of Therapy
- Shortest effective course based on infection type, response, toxicity.
- Surgical prophylaxis rarely >24 hours.
Route of Administration
- IV preferred for serious infections.
- Switch to oral when:
- Clinical and lab improvements are seen.
- No GI absorption issues.
- No severe immunosuppression.
Ancillary Measures
- Surgical drainage, foreign body removal, or obstruction relief may be needed.
- Biofilm on devices can hinder antibiotic effectiveness.
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