Lower respiratory tract infection acquired outside of healthcare settings.
Also known as: CAP, pneumonia, community-acquired pneumonia, bacterial pneumonia, lobar pneumonia
Overview
Acute infection of the lung parenchyma acquired outside of a hospital or healthcare facility, manifesting with new pulmonary infiltrate plus respiratory symptoms and signs of systemic infection.
Epidemiology
Leading infectious cause of death in the US. ~1.5 million ED visits annually. Streptococcus pneumoniae remains the most common identified bacterial pathogen; viruses (influenza, SARS-CoV-2, RSV) account for an increasing share.
Try two board-style Community-Acquired Pneumonia questions
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Question 1PulmonaryMedium
A previously healthy 50-year-old man presents with three days of fever, productive cough, and pleuritic chest pain. He has no comorbidities and is well enough for outpatient management. Examination and chest radiograph reveal right lower lobe consolidation, and a sputum Gram stain shows gram-positive diplococci. Which of the following is the most appropriate outpatient antibiotic?
ATrimethoprim-sulfamethoxazole (TMP-SMX)
BAzithromycin
CCiprofloxacin
DCephalexin
EAmoxicillin
Reveal answer & full explanation
Correct answer: E — Amoxicillin
ATrimethoprim-sulfamethoxazole (TMP-SMX)
BAzithromycin
CCiprofloxacin
DCephalexin
EAmoxicillin✓
Why Amoxicillin is correct
Gram-positive diplococci with lobar consolidation identify Streptococcus pneumoniae.
For a healthy outpatient with community-acquired pneumonia and no comorbidities, current IDSA/ATS guidance lists high-dose amoxicillin (or doxycycline) as preferred first-line therapy; a beta-lactam is the most reliable agent against documented pneumococcus.
Amoxicillin provides direct, high-level coverage of S. pneumoniae with a favorable safety and resistance profile.
Why the others are wrong
Azithromycin — Macrolide monotherapy is recommended only where local pneumococcal resistance is <25%, and U.S. resistance now exceeds that, so it is not preferred for documented pneumococcus; a buzzword-matching trap for 'CAP equals azithromycin.'
Trimethoprim-sulfamethoxazole (TMP-SMX) — Unreliable against S. pneumoniae and not a recommended CAP regimen; a wrong-spectrum trap.
Cephalexin — A first-generation cephalosporin with unreliable activity against penicillin-nonsusceptible S. pneumoniae and no place in current CAP guidance, which relies on the high serum exposure of high-dose amoxicillin; a wrong-beta-lactam trap.
Ciprofloxacin — A fluoroquinolone, which makes it look like a pneumonia drug, but ciprofloxacin is not a respiratory fluoroquinolone: its activity against S. pneumoniae is weak and unreliable, and it is not a recommended outpatient CAP regimen; a wrong-fluoroquinolone trap.
Question 2Infectious DiseaseEasy
A 60-year-old man hospitalized for pneumococcal pneumonia with penicillin-susceptible Streptococcus pneumoniae bacteremia has received intravenous ceftriaxone for 2 days. He is now afebrile, hemodynamically stable, and tolerating oral intake. Which of the following is the most appropriate step-down regimen and total duration of therapy?
AOral linezolid for 5-7 days
BOral azithromycin for 5 days
CIV ceftriaxone for 10-14 days
DOral ciprofloxacin for 7-10 days
EOral amoxicillin for 5-7 days
Reveal answer & full explanation
Correct answer: E — Oral amoxicillin for 5-7 days
AOral linezolid for 5-7 days
BOral azithromycin for 5 days
CIV ceftriaxone for 10-14 days
DOral ciprofloxacin for 7-10 days
EOral amoxicillin for 5-7 days✓
Why Oral amoxicillin for 5-7 days is correct
Once clinical stability is met (afebrile >24 h, heart rate <100, respiratory rate <24, improving oxygenation, tolerating oral intake), community-acquired pneumonia is stepped down to oral antibiotics
For penicillin-susceptible S. pneumoniae bacteremia, high-dose oral amoxicillin (1 g three times daily) is the appropriate agent
Total duration is 5-7 days per current IDSA/ATS community-acquired pneumonia guidance (minimum 5 days), driven by clinical (not microbiological) stability; the 2 days of IV ceftriaxone count toward that total, so about 3-5 more days of oral amoxicillin completes the course
Why the others are wrong
Oral linezolid for 5-7 days — reserved for resistant gram-positives (MRSA, VRE); using it here is over-coverage for a fully susceptible pneumococcus (buzzword-matching on 'bacteremia = broad coverage')
IV ceftriaxone for 10-14 days — continued IV therapy and prolonged duration are unnecessary once stability criteria are met (premature closure on 'finish the IV course')
Oral ciprofloxacin for 7-10 days — ciprofloxacin has poor pneumococcal activity and the duration exceeds guideline recommendations (right-class-wrong-drug trap; respiratory not urinary fluoroquinolone)
Oral azithromycin for 5 days — azithromycin is a familiar part of empiric community-acquired pneumonia regimens, but in hospitalized patients it is added to a beta-lactam for atypical coverage rather than used alone, and macrolide resistance is common among US pneumococci, so it is not appropriate pathogen-directed therapy for pneumococcal bacteremia (the trap is carrying an empiric regimen into culture-directed therapy)
Additional high-yield points
Clinical stability criteria, not repeat cultures, govern antibiotic duration in community-acquired pneumonia
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Pathogen reaches alveoli via microaspiration (most common), inhalation of droplets, or hematogenous spread. Overwhelms local defenses (mucociliary clearance, alveolar macrophages) → neutrophilic exudate fills alveoli → consolidation, impaired gas exchange. Common pathogens by setting: S. pneumoniae, H. influenzae, M. pneumoniae, C. pneumoniae, Legionella, S. aureus (post-influenza), respiratory viruses.
Clinical presentation
Symptoms
Cough (productive purulent sputum, or dry), fever, chills, rigors
Pleuritic chest pain, dyspnea
Constitutional: fatigue, myalgia, anorexia
Elderly may present atypically: confusion, falls, decompensation of chronic disease — without fever
Signs / physical exam
Fever, tachypnea, tachycardia, hypoxia
Focal crackles/rales, bronchial breath sounds over consolidation, egophony, increased tactile fremitus, dullness to percussion
Pleural friction rub if pleural involvement
Classic findings
Lobar consolidation with bronchial breath sounds and egophony classically points to S. pneumoniae; bullous myringitis with patchy infiltrates suggests Mycoplasma.
Differential diagnosis
Acute bronchitis — No infiltrate on CXR, mostly viral, productive cough with normal vitals
Pulmonary edema (cardiogenic vs ARDS) — Distribution and timing differ; clinical context, BNP, echocardiogram
Diagnostic workup
Labs
CBC (leukocytosis with left shift), BMP, lactate, procalcitonin (helps de-escalate antibiotics)
Blood cultures × 2 if severe, ICU admission, immunocompromise, or cavitary disease
Sputum Gram stain and culture if severe or specific pathogen suspected
Urine antigens: S. pneumoniae and Legionella (severe CAP, Legionella outbreak, travel)
Respiratory viral PCR (influenza, RSV, SARS-CoV-2) — especially seasonal
HIV testing for all newly diagnosed adults
Imaging
Chest radiograph (PA and lateral) — standard first test; chest imaging is REQUIRED to diagnose pneumonia; lobar consolidation, interstitial infiltrate, or cavitation
CT chest if non-resolving, complicated (abscess, empyema), or unclear
Bedside lung ultrasound — acceptable alternative to CXR for diagnosis where expertise exists (ATS 2025)
PSI/PORT score (more detailed, less practical at bedside; ATS/IDSA 2019 prefers PSI over CURB-65 for the hospitalization decision and uses the major/minor criteria below, not CURB-65, for ICU triage)
ATS/IDSA severe CAP criteria: 1 major (mechanical ventilation, septic shock with vasopressors) OR 3 minor (RR ≥30, PaO2/FiO2 ≤250, multilobar infiltrates, confusion, uremia, leukopenia, thrombocytopenia, hypothermia, hypotension requiring aggressive fluid)
Diagnostic algorithm
CURB-65 Score
30-Day Mortality
Disposition
0-1
<3%
Outpatient
2
~9%
Short inpatient stay or supervised outpatient
3-5
~15-57%
Inpatient; consider ICU
CURB-65 score (Confusion, Urea >19 mg/dL, RR ≥30, SBP <90 or DBP ≤60, age ≥65) guides disposition in CAP.
Treatment
First-line
Outpatient, no comorbidities, no recent antibiotics: amoxicillin 1 g TID OR doxycycline 100 mg BID OR macrolide (azithromycin, clarithromycin) only if local pneumococcal macrolide resistance is <25% (most US regions exceed this, so amoxicillin or doxycycline is usually preferred)
Outpatient with comorbidities (heart, lung, liver, renal, diabetes, alcohol, malignancy, asplenia; immunosuppressed patients such as those on biologics, chemotherapy, or chronic steroids also do not qualify for the healthy-outpatient regimens): beta-lactam (amoxicillin-clavulanate 875 mg BID or cefpodoxime/cefuroxime) PLUS macrolide or doxycycline; OR respiratory fluoroquinolone monotherapy (levofloxacin 750 mg, moxifloxacin 400 mg)
Inpatient (severe/ICU): beta-lactam + macrolide; OR beta-lactam + fluoroquinolone (use both, not monotherapy in severe)
Add MRSA coverage (vancomycin or linezolid) if prior MRSA isolate, recent hospitalization with IV antibiotics, or severe necrotizing pneumonia
Add Pseudomonas coverage (piperacillin-tazobactam, cefepime, meropenem) if prior Pseudomonas isolate or risk factors (bronchiectasis, structural lung disease)
Influenza-positive: oseltamivir 75 mg BID × 5 days (start within 48 h; later if hospitalized)
Second-line / adjunct
Duration: nonsevere CAP (outpatient or ward) that reaches clinical stability (afebrile, normal vitals, tolerating PO): less than 5 days (minimum 3 days); severe CAP: at least 5 days (ATS 2025). Do not stop before clinical stability
Adjunctive corticosteroids: suggested in severe CAP (CAPE-COD trial: hydrocortisone 200 mg/day reduced mortality) EXCEPT influenza pneumonia; do not give corticosteroids in nonsevere CAP (ATS 2025)
Drainage of complicated parapneumonic effusion or empyema (chest tube ± intrapleural tPA/DNase)
Complications
Parapneumonic effusion, empyema (Light criteria for exudate; pH <7.2 or pus = chest tube)
Post-pneumonia decline: cognitive, functional, cardiovascular events (MI, stroke) for months after
PANCE pearls
ATS/IDSA 2019 retired the 'healthcare-associated pneumonia (HCAP)' category — risk-stratify for MRSA and Pseudomonas individually instead.
Procalcitonin should NOT be used to withhold antibiotics in suspected CAP; use to guide de-escalation/duration once started.
Repeat CXR is not routinely needed for clinically responding patients; obtain at 6 weeks if smoker ≥50 to exclude underlying malignancy.
S. aureus pneumonia after influenza — cover empirically with vancomycin or linezolid in severely ill post-flu patients.
Legionella clues: GI symptoms, hyponatremia, elevated LFTs, relative bradycardia, recent travel/hotel/cruise; urine antigen tests for serogroup 1 only.
Mycoplasma (walking pneumonia): adolescents and young adults in close quarters (dorms, barracks, schools), gradual onset, dry cough, headache, low-grade fever, and a CXR that looks worse than the patient; cold agglutinins (IgM) occur in about half of cases and can cause hemolysis; bullous myringitis may be present; treat with a macrolide or doxycycline (beta-lactams do not work because it lacks a cell wall).
References
ATS/IDSA 2019 — Diagnosis and Treatment of Adults with Community-acquired Pneumonia (Metlay et al., Am J Respir Crit Care Med 2019)
CAPE-COD Trial — Hydrocortisone in Severe Community-Acquired Pneumonia (Dequin et al., NEJM 2023)
CURB-65 — Defining Community Acquired Pneumonia Severity on Presentation (Lim et al., Thorax 2003)
MIST2 Trial — Intrapleural Use of Tissue Plasminogen Activator and DNase in Pleural Infection (Rahman et al., NEJM 2011)
ATS 2025 — Diagnosis and Management of Community-acquired Pneumonia: An Official American Thoracic Society Clinical Practice Guideline (Jones et al., Am J Respir Crit Care Med 2026;212(1):24-44; published online July 2025)
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