What are the IDSA evidence‑based recommendations for selecting, dosing, and duration of antibiotics for community‑acquired pneumonia, skin and soft‑tissue infections, and uncomplicated urinary tract infection in adults?

IDSA Guidelines for Antibiotic Prescribing in Adults

Community-Acquired Pneumonia (CAP)

Outpatient Management – Previously Healthy Adults

Amoxicillin 1 g orally three times daily for 5–7 days is the preferred first-line therapy for previously healthy adults without comorbidities, providing superior pneumococcal coverage against 90–95% of Streptococcus pneumoniae isolates, including many penicillin-resistant strains. 1

• Doxycycline 100 mg orally twice daily for 5–7 days serves as an acceptable alternative when amoxicillin is contraindicated. 1
• Macrolide monotherapy (azithromycin or clarithromycin) should only be used when local pneumococcal macrolide resistance is documented to be <25%; in most U.S. regions, resistance is 20–30%, making macrolides unsafe as first-line therapy. 1

Outpatient Management – Adults with Comorbidities

For patients with chronic heart, lung, liver, or renal disease, diabetes, malignancy, or recent antibiotic use within 90 days, combination therapy is required. 1

• Option 1: Amoxicillin-clavulanate 875/125 mg orally twice daily plus azithromycin (500 mg day 1, then 250 mg daily for days 2–5) or doxycycline 100 mg twice daily. 1
• Option 2: Respiratory fluoroquinolone monotherapy—levofloxacin 750 mg daily or moxifloxacin 400 mg daily for 5–7 days—reserved for β-lactam allergy or when combination therapy is contraindicated. 1

Hospitalized Non-ICU Patients

Ceftriaxone 1–2 g IV once daily plus azithromycin 500 mg IV or orally daily is the standard regimen for hospitalized patients not requiring ICU care, providing coverage for typical bacteria and atypical pathogens. 1

• Alternative β-lactams include cefotaxime 1–2 g IV every 8 hours or ampicillin-sulbactam 3 g IV every 6 hours, always combined with azithromycin. 1
• Respiratory fluoroquinolone monotherapy (levofloxacin 750 mg IV daily or moxifloxacin 400 mg IV daily) is an equally effective alternative, particularly for penicillin-allergic patients. 1
• Administer the first antibiotic dose in the emergency department immediately upon diagnosis; delays beyond 8 hours increase 30-day mortality by 20–30%. 1

ICU Patients (Severe CAP)

Combination therapy is mandatory for all ICU patients; β-lactam monotherapy is associated with higher mortality in critically ill individuals. 1

• Preferred regimen: Ceftriaxone 2 g IV once daily (or cefotaxime 1–2 g IV every 8 hours or ampicillin-sulbactam 3 g IV every 6 hours) plus azithromycin 500 mg IV daily or a respiratory fluoroquinolone (levofloxacin 750 mg IV daily or moxifloxacin 400 mg IV daily). 1
• For penicillin-allergic ICU patients, use aztreonam 2 g IV every 8 hours plus a respiratory fluoroquinolone. 1

Special Pathogen Coverage

Antipseudomonal coverage should be added only when specific risk factors are present: structural lung disease (bronchiectasis, cystic fibrosis), recent hospitalization with IV antibiotics within 90 days, or prior respiratory isolation of Pseudomonas aeruginosa. 1

• Regimen: Piperacillin-tazobactam 4.5 g IV every 6 hours or cefepime 2 g IV every 8 hours plus ciprofloxacin 400 mg IV every 8 hours or levofloxacin 750 mg IV daily plus an aminoglycoside (gentamicin or tobramycin 5–7 mg/kg IV daily) for dual antipseudomonal coverage. 1

MRSA coverage is indicated only when risk factors are present: prior MRSA infection/colonization, recent hospitalization with IV antibiotics, post-influenza pneumonia, or cavitary infiltrates on imaging. 1

• Regimen: Vancomycin 15 mg/kg IV every 8–12 hours (target trough 15–20 µg/mL) or linezolid 600 mg IV every 12 hours, added to the base CAP regimen. 1

Duration and Transition to Oral Therapy

Treat for a minimum of 5 days and continue until the patient is afebrile for 48–72 hours with no more than one sign of clinical instability. 1, 2

• Typical duration for uncomplicated CAP is 5–7 days. 1, 2
• Extended courses of 14–21 days are required only for Legionella pneumophila, Staphylococcus aureus, or Gram-negative enteric bacilli. 1, 2
• Switch from IV to oral therapy when the patient is hemodynamically stable (systolic BP ≥90 mmHg, heart rate ≤100 bpm), clinically improving, afebrile for 48–72 hours, respiratory rate ≤24 breaths/min, oxygen saturation ≥90% on room air, and able to take oral medication—typically by hospital day 2–3. 1, 2

Critical Pitfalls to Avoid

• Never use macrolide monotherapy in hospitalized patients; it fails to cover typical pathogens like S. pneumoniae and leads to treatment failure. 1
• Avoid indiscriminate fluoroquinolone use in uncomplicated outpatient CAP due to FDA warnings about serious adverse events (tendon rupture, peripheral neuropathy, aortic dissection) and rising resistance. 1
• Do not add broad-spectrum antipseudomonal or MRSA agents routinely; restrict to patients with documented risk factors to prevent resistance and adverse effects. 1
• Obtain blood and sputum cultures before initiating antibiotics in all hospitalized patients to enable pathogen-directed therapy and safe de-escalation. 1, 3

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Skin and Soft Tissue Infections (SSTIs)

Uncomplicated SSTIs

For simple cellulitis or abscesses without systemic signs of infection, oral antibiotics targeting Staphylococcus aureus and Streptococcus species are appropriate. 4

• Preferred agents: Cephalexin 500 mg orally four times daily or dicloxacillin 500 mg orally four times daily for 5–7 days. 4
• For suspected MRSA (community-acquired strains), use trimethoprim-sulfamethoxazole 1–2 double-strength tablets orally twice daily or doxycycline 100 mg orally twice daily. 4
• Incision and drainage is the primary treatment for abscesses; antibiotics are adjunctive. 4

Complicated SSTIs (cSSTIs)

Complicated SSTIs require hospitalization and IV antibiotics when deep-seated infection, surgical intervention, systemic sepsis, complicating comorbidities, or tissue necrosis is present. 4, 5

• For polymicrobial infections (diabetic foot infections, perirectal abscesses, burns): Use β-lactam/β-lactamase inhibitor combinations such as ampicillin-sulbactam 3 g IV every 6 hours or piperacillin-tazobactam 3.375 g IV every 6 hours. 4
• For MRSA cSSTI: Vancomycin 15 mg/kg IV every 8–12 hours (target trough 15–20 µg/mL) or linezolid 600 mg IV every 12 hours or daptomycin 4–6 mg/kg IV daily. 4
• Prompt surgical debridement or drainage is essential for successful management; antibiotics alone are insufficient for necrotizing infections or abscesses. 4

Duration of Therapy

• Uncomplicated SSTIs: 5–7 days of oral antibiotics. 4
• Complicated SSTIs: 7–14 days depending on clinical response, with transition to oral therapy when clinically stable. 4, 5

Common Pitfalls

• Do not delay surgical intervention in necrotizing fasciitis or large abscesses; antibiotics cannot substitute for source control. 4
• Inappropriate initial antibiotic therapy is associated with worse outcomes in culture-positive cSSTI, particularly when MRSA or mixed pathogens are involved. 5

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Uncomplicated Urinary Tract Infections (UTIs) in Women

Acute Uncomplicated Cystitis

Nitrofurantoin monohydrate/macrocrystals 100 mg orally twice daily for 5 days is the preferred first-line agent for acute uncomplicated cystitis in women, minimizing collateral damage to gut and vaginal flora. 6

• Alternative agents: Trimethoprim-sulfamethoxazole 160/800 mg (one double-strength tablet) orally twice daily for 3 days, only if local resistance is <20%. 6
• Fosfomycin 3 g orally as a single dose is an acceptable alternative but has slightly lower efficacy. 6
• Fluoroquinolones (ciprofloxacin, levofloxacin) should be reserved for pyelonephritis or complicated UTIs due to resistance concerns and adverse effects. 6

Acute Uncomplicated Pyelonephritis

For outpatient management of mild pyelonephritis, ciprofloxacin 500 mg orally twice daily for 7 days (or 1000 mg extended-release once daily for 7 days) or levofloxacin 750 mg orally once daily for 5 days is recommended. 6

• If fluoroquinolone resistance is >10% in the community, give a one-time dose of ceftriaxone 1 g IV or an aminoglycoside before starting oral fluoroquinolone therapy. 6
• For hospitalized patients with pyelonephritis: Ceftriaxone 1–2 g IV once daily or an aminoglycoside (gentamicin 5–7 mg/kg IV daily) until afebrile for 24–48 hours, then switch to oral fluoroquinolone to complete 10–14 days total. 6

Duration of Therapy

• Acute cystitis: 3–5 days depending on agent. 6
• Acute pyelonephritis: 7–14 days depending on severity and clinical response. 6

Critical Pitfalls

• Avoid fluoroquinolones for simple cystitis; reserve for pyelonephritis or complicated UTIs to preserve efficacy and minimize resistance. 6
• Do not use trimethoprim-sulfamethoxazole empirically if local resistance exceeds 20%; obtain urine culture and adjust therapy based on susceptibility. 6
• Obtain urine culture before initiating antibiotics in all women with suspected pyelonephritis to guide targeted therapy. 6