What are the recommended antibiotic regimens for a patient with no significant medical history and normal renal and hepatic function, presenting with community-acquired pneumonia, uncomplicated urinary tract infections, or skin infections?

Antibiotic Selection for Community-Acquired Pneumonia, Uncomplicated UTI, and Skin Infections

Community-Acquired Pneumonia (CAP)

Outpatient Treatment - Healthy Adults Without Comorbidities

Amoxicillin 1 gram orally three times daily for 5-7 days is the preferred first-line antibiotic for previously healthy adults with community-acquired pneumonia. 1, 2

• Doxycycline 100 mg orally twice daily serves as an acceptable alternative if amoxicillin cannot be tolerated 1, 2
• Macrolides (azithromycin 500 mg day 1, then 250 mg daily for days 2-5) should only be used when local pneumococcal macrolide resistance is documented to be <25% 1, 2
• Macrolide monotherapy should be avoided in areas with ≥25% resistance due to high treatment failure rates 1, 2

Outpatient Treatment - Adults With Comorbidities

For patients with comorbidities (COPD, diabetes, heart/lung/liver/renal disease, alcoholism, malignancy), combination therapy is mandatory. 1, 2

Recommended regimen: Amoxicillin-clavulanate 875 mg/125 mg orally twice daily PLUS azithromycin 500 mg day 1, then 250 mg daily for days 2-5, total duration 5-7 days 1, 2

• Alternative β-lactams include cefpodoxime or cefuroxime if amoxicillin-clavulanate is not tolerated 1, 2
• Alternative monotherapy: Levofloxacin 750 mg orally once daily for 5 days OR moxifloxacin 400 mg orally once daily for 5 days 1, 2, 3
• If the patient used antibiotics within the past 90 days, select an agent from a different antibiotic class to reduce resistance risk 1, 2

Inpatient Treatment - Non-ICU Patients

Two equally effective regimens exist with strong evidence: 1, 2

1. Ceftriaxone 1-2 grams IV once daily PLUS azithromycin 500 mg IV or oral daily 1, 2, 4
2. Respiratory fluoroquinolone monotherapy: Levofloxacin 750 mg IV once daily OR moxifloxacin 400 mg IV once daily 1, 2, 3

• Administer the first antibiotic dose immediately in the emergency department, as delayed administration beyond 8 hours increases 30-day mortality by 20-30% 1, 2
• Switch from IV to oral therapy when hemodynamically stable, clinically improving, afebrile for 48-72 hours, and able to take oral medications—typically by day 2-3 1, 2
• For penicillin-allergic patients, use respiratory fluoroquinolone as the preferred alternative 1, 2

Inpatient Treatment - ICU/Severe CAP

Combination therapy is mandatory for all ICU patients—monotherapy is inadequate for severe disease. 1, 2

Preferred regimen: Ceftriaxone 2 grams IV once daily PLUS azithromycin 500 mg IV daily OR levofloxacin 750 mg IV once daily 1, 2

• Alternative β-lactams: Cefotaxime 1-2 grams IV every 8 hours OR ampicillin-sulbactam 3 grams IV every 6 hours 1, 2
• Duration: 10-14 days for severe CAP, extending to 14-21 days if Legionella pneumophila, Staphylococcus aureus, or Gram-negative enteric bacilli are identified 1, 2, 5

Special Populations Requiring Broader Coverage

Add antipseudomonal coverage ONLY if: 1, 2

• Structural lung disease (bronchiectasis)
• Recent hospitalization with IV antibiotics within 90 days
• Prior respiratory isolation of Pseudomonas aeruginosa

Antipseudomonal regimen: Piperacillin-tazobactam 4.5 grams IV every 6 hours OR cefepime 2 grams IV every 8 hours PLUS ciprofloxacin 400 mg IV every 8 hours OR levofloxacin 750 mg IV daily PLUS aminoglycoside (gentamicin 5-7 mg/kg IV daily) 6, 1, 2

Add MRSA coverage ONLY if: 1, 2

• Prior MRSA infection or colonization
• Recent hospitalization with IV antibiotics
• Post-influenza pneumonia
• Cavitary infiltrates on imaging

MRSA regimen: Add vancomycin 15 mg/kg IV every 8-12 hours (target trough 15-20 mg/mL) OR linezolid 600 mg IV every 12 hours to the base regimen 6, 1, 2, 7

Duration and Monitoring

• Standard duration: Minimum 5 days and until afebrile for 48-72 hours with no more than one sign of clinical instability; typical duration 5-7 days for uncomplicated CAP 1, 2
• Extended duration (14-21 days): Required for Legionella, Staphylococcus aureus, or Gram-negative enteric bacilli 1, 2, 5
• Assess clinical response at day 2-3; if no improvement, obtain repeat chest radiograph, CRP, white cell count, and additional cultures 6, 1

Critical Pitfalls to Avoid

• Never use macrolide monotherapy in hospitalized patients or those with comorbidities—provides inadequate coverage for typical bacterial pathogens 1, 2
• Never use macrolide monotherapy in areas where pneumococcal resistance exceeds 25%—leads to treatment failure 1, 2
• Avoid indiscriminate fluoroquinolone use in uncomplicated outpatient CAP due to FDA warnings about serious adverse events (tendinopathy, peripheral neuropathy, CNS effects) and resistance concerns 1, 3
• Do not automatically escalate to broad-spectrum antibiotics without documented risk factors for resistant organisms 1, 2

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Uncomplicated Urinary Tract Infections (UTI)

First-Line Treatment for Uncomplicated Cystitis

For simple uncomplicated cystitis in women, use short-course therapy with high efficacy and minimal resistance: 6, 8

• Nitrofurantoin 100 mg orally every 6 hours for 5-7 days 6
• Fosfomycin 3 grams orally as a single dose 6
• Fluoroquinolones (if other options unavailable): Ciprofloxacin 250 mg orally twice daily for 3 days OR levofloxacin 250 mg orally once daily for 3 days 3, 8

Treatment Considerations

• Fluoroquinolones should be reserved for patients when other options cannot be used due to resistance concerns and serious adverse events 3
• For patients with recent antibiotic exposure, select an agent from a different class 1
• No dose adjustment required for nitrofurantoin or fosfomycin in normal renal function 6

Complicated UTI or Pyelonephritis

For complicated UTI or acute pyelonephritis: 6, 3

• Levofloxacin 750 mg orally or IV once daily for 5-7 days 3
• Ciprofloxacin 500 mg orally twice daily OR 400 mg IV every 12 hours for 7-10 days 6
• Obtain urine culture before initiating antibiotics in all hospitalized patients 6

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Skin and Soft Tissue Infections

Uncomplicated Cellulitis/Erysipelas

For uncomplicated cellulitis without purulence or systemic signs of infection: 6

• Cephalexin 500 mg orally four times daily for 5-7 days (covers Streptococcus pyogenes and methicillin-sensitive Staphylococcus aureus)
• Dicloxacillin 500 mg orally four times daily for 5-7 days (alternative for penicillin-tolerant patients)
• Clindamycin 300-450 mg orally three times daily for 5-7 days (for penicillin-allergic patients)

Purulent Skin Infections (Abscesses, Furuncles)

For purulent skin infections with suspected community-acquired MRSA: 6

• Incision and drainage is the primary treatment—antibiotics are adjunctive 6
• Trimethoprim-sulfamethoxazole 1-2 double-strength tablets orally twice daily for 5-7 days (first-line for CA-MRSA)
• Doxycycline 100 mg orally twice daily for 5-7 days (alternative for CA-MRSA)
• Clindamycin 300-450 mg orally three times daily for 5-7 days (alternative, but resistance rates vary)

Complicated Skin Infections Requiring Hospitalization

For complicated skin infections with systemic signs or failed outpatient therapy: 6

• Vancomycin 15 mg/kg IV every 8-12 hours (target trough 15-20 mg/mL) for MRSA coverage 6
• Linezolid 600 mg IV or oral every 12 hours for MRSA coverage 6, 7
• Ceftriaxone 1-2 grams IV once daily PLUS vancomycin for mixed infections 6

Special Considerations

• Obtain MRSA PCR for purulent skin infections in settings with high CA-MRSA prevalence to guide targeted therapy 6
• Blood cultures are generally not indicated for uncomplicated cellulitis unless immunocompromised, intravascular prosthesis, or high-risk features present 6
• Duration: 5-7 days for uncomplicated infections; extend to 10-14 days for complicated infections or slow clinical response 6

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Renal Function Considerations

For patients with renal impairment (GFR 67 mL/min as in your example): 9, 10

• Amoxicillin, amoxicillin-clavulanate, ceftriaxone: No dose adjustment required for mild-moderate renal impairment 1, 9
• Azithromycin: No dose adjustment required for any degree of renal impairment 4, 9
• Levofloxacin: Dose adjustment required—use 750 mg loading dose, then 500 mg every 48 hours for GFR 20-49 mL/min 3, 9
• Nitrofurantoin: Avoid if GFR <60 mL/min due to reduced efficacy and increased toxicity risk 9
• Loading doses: Always administer full loading doses regardless of renal function; only maintenance doses require adjustment 10