Community-Acquired Pneumonia (CAP)
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.
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Risk factors
- Age ≥65 years or <2 years
- Smoking, alcohol use disorder
- Chronic lung disease (COPD, asthma, bronchiectasis), heart failure, diabetes, CKD, liver disease
- Immunocompromise (HIV, chemotherapy, immunosuppressants, asplenia)
- Recent viral URI (especially influenza — predisposes to secondary bacterial pneumonia)
- Aspiration risk (impaired swallow, altered mental status)
Pathophysiology
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 embolism — Pleuritic pain, hypoxia disproportionate to imaging, VTE risk factors, D-dimer/CTPA
- Heart failure exacerbation — Bilateral interstitial pattern, cardiomegaly, elevated BNP, orthopnea/PND
- Aspiration pneumonitis — Witnessed aspiration, rapid improvement (chemical), favors dependent segments
- Lung cancer with post-obstructive pneumonia — Non-resolving infiltrate, hemoptysis, weight loss, smoker; follow-up imaging required
- Tuberculosis — Subacute symptoms, hemoptysis, upper-lobe cavitary disease, exposure history, IGRA/AFB
- COVID-19 pneumonia — Bilateral peripheral ground-glass opacities, anosmia/ageusia, lymphopenia, positive PCR
- 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) — REQUIRED to diagnose pneumonia; lobar consolidation, interstitial infiltrate, or cavitation
- CT chest if non-resolving, complicated (abscess, empyema), or unclear
- Bedside lung ultrasound — increasingly used in EDs
Other studies
- Severity scores: CURB-65 (Confusion, Urea >19 mg/dL, RR ≥30, BP <90/60, age ≥65) — 0-1 outpatient, 2 ward, ≥3 ICU consideration
- PSI/PORT score (more detailed, less practical at bedside)
- 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 | 15-40% | Inpatient; consider ICU |
Treatment
First-line
- Outpatient, no comorbidities, no recent antibiotics: amoxicillin 1 g TID OR doxycycline 100 mg BID OR macrolide (azithromycin, clarithromycin) if local pneumococcal resistance <25%
- Outpatient with comorbidities (heart, lung, liver, renal, diabetes, alcohol, malignancy, asplenia): 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 (non-severe): beta-lactam (ceftriaxone 1-2 g, ampicillin-sulbactam, cefotaxime) + macrolide (azithromycin); OR respiratory fluoroquinolone monotherapy
- 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: minimum 5 days; continue until clinically stable (afebrile, normal vitals, tolerating PO) — most patients 5-7 days
- Adjunctive corticosteroids in severe CAP (CAPE-COD trial: hydrocortisone 200 mg/day) reduces mortality
- 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)
- Lung abscess (anaerobes, S. aureus, Klebsiella)
- Bacteremia, sepsis, septic shock
- ARDS, respiratory failure requiring mechanical ventilation
- 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.
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)
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