Antibiotic Selection for Suspected Infection
The appropriate antibiotic depends entirely on the suspected source of infection and patient risk factors—without knowing the infection site (respiratory, intra-abdominal, skin/soft tissue, etc.), I cannot recommend a specific agent, but I will provide a structured approach to guide selection based on the most common clinical scenarios.
General Principles for Antibiotic Selection
Before prescribing any antibiotic, you must identify:
- Infection source (respiratory, urinary, intra-abdominal, skin/soft tissue, bloodstream) 1, 2
- Patient risk factors for resistant organisms (recent antibiotics, healthcare exposure, known colonization) 3, 4
- Severity of illness (septic shock, organ dysfunction, immunocompromise) 5, 4
- Local resistance patterns in your institution 5
Obtain cultures before antibiotics if this does not significantly delay administration 5, 4. In septic patients, speed matters, but patient-specific characteristics should determine antibiotic breadth 4.
Community-Acquired Respiratory Infections
Lower Respiratory Tract Infections (Outpatient)
For community-acquired pneumonia or acute bronchitis in primary care, amoxicillin 500-1000 mg every 8 hours orally is the first-line choice 5.
Alternative regimens:
- Tetracycline or doxycycline 100 mg every 12 hours for penicillin allergy 5
- Macrolides (azithromycin 500 mg day 1, then 250 mg daily for 4 days; or clarithromycin 250-500 mg every 12 hours) in areas with low pneumococcal macrolide resistance 5
- Levofloxacin or moxifloxacin only when resistance rates to first-line agents are clinically significant 5
Duration: Minimum 7 days (except azithromycin/clarithromycin which have shorter courses) 5.
COPD Exacerbations
Antibiotics are indicated only when patients have all three: increased dyspnea, increased sputum volume, AND increased sputum purulence 5. Also consider antibiotics in severe COPD exacerbations regardless of symptoms 5.
Use the same agents as above (amoxicillin, tetracycline, or macrolides) 5.
Hospitalized Pneumonia
For medical ward patients: IV cefuroxime 750-1500 mg every 8 hours, ceftriaxone 1 g every 24 hours, or cefotaxime 1 g every 8 hours 5.
For ICU patients with severe pneumonia: combination therapy with a third-generation cephalosporin (cefotaxime) PLUS either a fluoroquinolone (ciprofloxacin/ofloxacin) OR macrolide (erythromycin 1 g every 6 hours IV) 5.
Intra-Abdominal Infections
Community-Acquired, Immunocompetent Patients
For mild-moderate community-acquired intra-abdominal infection: piperacillin-tazobactam, ertapenem 1 g every 24 hours, or a combination regimen 5.
Duration: 4 days if adequate source control achieved in immunocompetent patients; up to 7 days in immunocompromised or critically ill 5.
Septic Shock or Healthcare-Associated
For septic shock: meropenem 1 g every 6 hours by extended infusion, doripenem 500 mg every 8 hours by extended infusion, or imipenem/cilastatin 500 mg every 6 hours by extended infusion 5.
For suspected multidrug-resistant organisms: imipenem/cilastatin-relebactam 1.25 g every 6 hours, meropenem/vaborbactam 2 g/2 g every 8 hours, or ceftazidime/avibactam 2.5 g every 8 hours plus metronidazole 500 mg every 8 hours 5.
For beta-lactam allergy: eravacycline 1 mg/kg every 12 hours 5.
Skin and Soft Tissue Infections
Uncomplicated Cellulitis/Abscess (Outpatient)
For suspected methicillin-sensitive S. aureus: dicloxacillin 500 mg orally four times daily or cephalexin 500 mg orally three times daily 6, 3.
For MRSA risk factors (recent antibiotics, known colonization, injection drug use, failed beta-lactam therapy): trimethoprim-sulfamethoxazole 160-800 mg twice daily, doxycycline 100 mg twice daily, or clindamycin 300 mg three times daily 6, 3.
Severe/Hospitalized Infections
For MSSA: nafcillin or oxacillin 1-2 g every 4-6 hours IV, or cefazolin 1 g every 8 hours IV 3.
For MRSA: vancomycin 15-20 mg/kg every 8-12 hours IV (target trough 15-20 mcg/mL) 6, 3.
For necrotizing infection: vancomycin PLUS piperacillin-tazobactam or a carbapenem, with urgent surgical consultation 7, 3.
Febrile Neutropenia
High-Risk Patients
High-risk patients (ANC <100 cells/mm³ for >7 days, significant comorbidities) require hospitalization and IV monotherapy with cefepime, meropenem, imipenem-cilastatin, or piperacillin-tazobactam 5.
Do NOT routinely add vancomycin unless there is suspected catheter-related infection, skin/soft tissue infection, pneumonia, or hemodynamic instability 5.
Low-Risk Patients (MASCC Score ≥21)
Low-risk patients can receive oral ciprofloxacin plus amoxicillin-clavulanate 5.
Critical Considerations and Pitfalls
Penicillin Allergy
Most patients labeled as penicillin-allergic are not truly allergic 8. However, for documented immediate-type hypersensitivity reactions:
- Respiratory infections: Use fluoroquinolones or macrolides 5
- Intra-abdominal infections: Use eravacycline or fluoroquinolone-based regimens 5
- Skin infections: Use clindamycin, vancomycin, or doxycycline 6, 3
Avoid carbapenems in patients with immediate penicillin hypersensitivity 5.
Vancomycin + Piperacillin-Tazobactam Combination
This combination is associated with increased acute kidney injury and should be avoided unless specifically indicated for polymicrobial necrotizing infections 7, 4.
Enterococcal Coverage
Do not routinely cover enterococci in community-acquired infections 5. Reserve anti-enterococcal therapy for healthcare-associated intra-abdominal infections, particularly post-operative infections or in immunocompromised patients 5.
Monitoring and De-escalation
Expect clinical improvement within 2-3 days 5. If no improvement by day 3, reassess diagnosis, obtain cultures, and consider resistant organisms or inadequate source control 5.
Patients should return if symptoms persist beyond 3 weeks, fever exceeds 4 days, dyspnea worsens, or consciousness decreases 5.