Community-acquired pneumonia (outpatient): empiric antibiotic therapy

ATS/IDSA2019

Healthy outpatients, no comorbidities: amoxicillin 1 g PO TID (strong, moderate-quality evidence), OR doxycycline 100 mg PO BID (conditional, low-quality), OR a macrolide (azithromycin 500 mg x1 then 250 mg daily, or clarithromycin 500 mg BID) only where local pneumococcal macrolide resistance is under 25% (conditional, moderate-quality). Treat until clinically stable; minimum duration historically 5 days. A 2025 ATS-only update (not approved by IDSA) suggests a shorter course of 3 to under 5 days once clinically stable.

ATS/IDSA2019

Outpatients with comorbidities (chronic heart/lung/liver/renal disease, diabetes, alcoholism, malignancy, asplenia): combination of amoxicillin/clavulanate (875/125 mg BID) or a cephalosporin (cefpodoxime 200 mg BID, cefuroxime 500 mg BID) PLUS a macrolide or doxycycline 100 mg BID; OR fluoroquinolone monotherapy: levofloxacin 750 mg, moxifloxacin 400 mg, or gemifloxacin 320 mg daily. Regimen choices unchanged in the 2025 ATS update, which defers antibiotic selection to the 2019 guideline.

ATS/IDSA2019Grade Strong recommendation (PSI, cultures, procalcitonin)

Decide outpatient versus inpatient care with clinical judgment plus a validated rule, preferably the Pneumonia Severity Index (strong) over CURB-65 (conditional). In outpatients, do not routinely obtain sputum Gram stain and culture or blood cultures (strong). Start antibiotics for clinically suspected, radiographically confirmed CAP regardless of procalcitonin (strong). If symptoms resolve within 5 to 7 days, do not routinely repeat chest imaging (conditional).

ATS2025Grade Conditional recommendation, very low-quality evidence

Outpatients without comorbidities who have clinical and imaging evidence of CAP and test positive for a respiratory virus: suggest not prescribing empiric antibiotics. Lung ultrasound is an acceptable alternative to chest x-ray where the clinical expertise exists (conditional, low-quality evidence). This is an ATS-only update of the 2019 guideline; IDSA did not endorse it.