What are the guidelines for choosing antibiotics in patients?

Antibiotic Selection Guidelines for Patients

Antibiotic selection should be guided by the site of infection, severity of illness, risk factors for resistant organisms, local resistance patterns, and appropriate microbiological sampling before treatment initiation. 1

Core Principles for Antibiotic Selection

Initial Assessment and Microbiological Sampling

• Obtain appropriate cultures before starting antibiotics in all hospitalized patients, including blood cultures (two sets) for suspected bloodstream infections, sputum cultures for respiratory infections, and site-specific cultures as indicated 2, 3
• Start empirical therapy promptly after obtaining cultures in septic or severely ill patients, as delays increase mortality 1
• Colonization without clinical signs of infection rarely requires treatment 3

Risk Stratification for Empirical Therapy

High-risk patients requiring broad-spectrum coverage include those with: 2, 4

• Severe sepsis or hemodynamic instability
• Recent hospitalization (within 90 days)
• Recent antibiotic exposure (within 3 months)
• Known colonization with resistant organisms (MRSA, ESBL-producing organisms)
• Immunocompromised status
• Healthcare-associated infections

Low-risk patients with community-acquired infections can typically be managed with traditional first-line antibiotics 4

Infection-Specific Antibiotic Recommendations

Skin and Soft Tissue Infections

For methicillin-susceptible S. aureus (MSSA): 2

• Oral therapy: Dicloxacillin 500 mg four times daily or cephalexin 500 mg four times daily
• Parenteral therapy: Nafcillin or oxacillin 1-2 g every 4 hours IV, or cefazolin 1 g every 8 hours IV

For methicillin-resistant S. aureus (MRSA): 2

• Parenteral first-line: Vancomycin 30 mg/kg/day in 2 divided doses IV
• Oral alternatives: Linezolid 600 mg twice daily, doxycycline 100 mg twice daily, or TMP-SMZ 1-2 double-strength tablets twice daily
• Clindamycin 300-450 mg three times daily is an option but has potential for inducible resistance

For impetigo (limited lesions): 2

• Mupirocin ointment applied three times daily
• Oral options: Dicloxacillin 250 mg four times daily, cephalexin 250 mg four times daily, or clindamycin 300-400 mg three times daily

Lower Respiratory Tract Infections

Community-acquired pneumonia (non-severe, hospitalized): 2

• Preferred regimen: Amoxicillin 500-1000 mg three times daily PLUS a macrolide (erythromycin or clarithromycin)
• Alternative for penicillin allergy: Levofloxacin (only fluoroquinolone currently recommended in UK guidelines)
• Most patients can be treated with oral antibiotics 2

Community-acquired pneumonia (severe, ICU): 2

• Parenteral combination therapy with IV beta-lactam PLUS macrolide
• Consider broader coverage for Pseudomonas if risk factors present

COPD exacerbations requiring antibiotics (Anthonisen Type I - all three symptoms: increased dyspnea, sputum volume, and purulence): 2

• First-line: Amoxicillin-clavulanate (co-amoxiclav)
• Alternatives: Levofloxacin or moxifloxacin (though WHO 2024 guidelines recommend avoiding fluoroquinolones due to side effects) 2
• For P. aeruginosa risk (≥2 factors: recent hospitalization, frequent antibiotics >4 courses/year, FEV1 <30%, oral steroids >10 mg/day): Ciprofloxacin oral, or IV ciprofloxacin/antipseudomonal beta-lactam 2

COPD exacerbations NOT requiring antibiotics: 2

• Anthonisen Type II without purulence (only 2 symptoms, purulence not one of them)
• Anthonisen Type III (only 1 symptom)

Specific Clinical Scenarios

Febrile neutropenia (high-risk patients): 2

• Monotherapy: Cefepime, ceftazidime, meropenem, or imipenem-cilastatin IV
• Add vancomycin if: Clinically suspected catheter-related infection, known MRSA/resistant pneumococcal colonization, positive blood culture for gram-positive bacteria, or hypotension
• Low-risk adults may receive oral ciprofloxacin plus amoxicillin-clavulanate 2

Diabetic foot infections: 1

• Treat all clinically infected wounds; do not treat uninfected wounds
• Consider empiric MRSA coverage if prior MRSA history, high local prevalence, or severe infection

De-escalation and Duration

Streamlining Therapy

• Switch from IV to oral by day 3 if patient is clinically stable 2
• De-escalate based on culture results and clinical response within 3-5 days 2, 1, 3
• Discontinue antibiotics when infection is unlikely or ruled out 1, 3
• Median time to defervescence in neutropenic patients is 5 days; reassess if no improvement by day 3-5 2

Duration of Therapy

• Impetigo: Approximately 7 days depending on clinical response 2
• Streptococcal infections: Continue for 10 days 5
• Uncomplicated gonococcal/chlamydial infections: 7 days 5
• Early syphilis (penicillin allergy): Doxycycline 100 mg twice daily for 2 weeks 5

Critical Pitfalls to Avoid

Common prescribing errors: 1, 3

• Treating fever without evidence of bacterial infection
• Using antibiotics for viral infections (including mild-moderate COVID-19 without bacterial superinfection)
• Failing to obtain cultures before starting therapy
• Continuing antibiotics when cultures are negative and clinical infection is unlikely
• Using broad-spectrum agents when narrow-spectrum options are adequate

Resistance considerations: 2, 3

• Avoid fluoroquinolones as first-line agents due to serious side effects (tendon, muscle, joint problems, peripheral neuropathy) and resistance concerns
• Minimize use of third-generation cephalosporins and fluoroquinolones to reduce selection pressure for resistant organisms
• Select different antibiotic classes in patients with recent antibiotic exposure 1

Special populations: 1, 6, 7

• Exercise caution with azithromycin in hepatic impairment 6
• Adjust dosing for renal impairment based on creatinine clearance
• Take amoxicillin-clavulanate with food to reduce GI upset 7
• Doxycycline not recommended for children <8 years 2, 5