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Community-Acquired Pneumonia (CAP)

Definition

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Epidemiology

• ~3 million cases and 1.6 million hospitalizations annually in the US
• ~7 health care visits/1,000 per year in young adults vs. 96/1,000 in adults ≥85 years
• Hospitalization rate: 1–2/1,000 in young adults → 15–20/1,000 in ages 65–84 → ~40/1,000 in ≥85 years
• ~35% of CAP hospitalizations occur in patients with recent healthcare exposures
• ~25% occur in immunocompromised patients
• Mortality: ~6% during hospitalization; ~15% at 1 month for hospitalized CAP
• Seasonal peaks November–March (influenza, RSV, SARS-CoV-2)

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Pathobiology

Routes of Infection

Host Defense Disruption

• Mucociliary clearance (ciliated epithelium + mucous layer)
• Cough reflex
• Bacteriostatic surfactant lining the alveoli
• Innate and adaptive immune responses (alveolar macrophages, neutrophils)
• Lung microbiome homeostasis (Prevotella, Veillonella, Streptococcus species in healthy lung)

• Increased oropharyngeal colonization (poor dental hygiene, alcohol use)
• Decreased cough/ciliary function (neurologic disease, COPD, cystic fibrosis)
• Immunosuppression (corticosteroids, chemotherapy, HIV, malnutrition)
• Structural lung disease (bronchiectasis)
• Aging (loss of stem cell reservoirs, mitochondrial dysfunction, upregulated bacterial adhesion receptors)
• Medications: opioids, PPIs, corticosteroids

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Causative Pathogens

By Patient Setting (in order of decreasing frequency)

Overall: Pneumococcus, H. influenzae, S. aureus, and GNRs cause ~30% of cases; Mycoplasma, Chlamydia, Legionella <5%; respiratory viruses 20–30% (higher post-COVID); no pathogen identified in >50% of cases.

Risk Factors for Specific Pathogens

• DRSP: Age >65, β-lactam use within 3 months, alcoholism, corticosteroids, multiple comorbidities, child in daycare
• Enteric GNRs / P. aeruginosa: Nursing home, underlying cardiopulmonary disease, corticosteroids >10 mg/day prednisone, broad-spectrum antibiotics >7 days in past month, structural lung disease (bronchiectasis), malnutrition
• MRSA / MDR organisms: Hospitalization ≥2 days in past 90 days, broad-spectrum antibiotics in past 3 months, immunosuppression, poor functional status, prior respiratory isolation of MRSA or P. aeruginosa (key 2019 guideline update)
• Anaerobes: Poor dental hygiene (aspiration pneumonia)

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Clinical Manifestations

Symptoms

• Cough (with or without purulent sputum)
• Fever, chills, rigors
• Pleuritic chest pain
• Dyspnea, tachypnea
• Fatigue, myalgias, headache (more common in atypical/viral)

Signs

• Tachypnea (RR >20), tachycardia
• Fever (may be absent in elderly)
• Decreased breath sounds, dullness to percussion
• Bronchial breath sounds, egophony, whispered pectoriloquy over consolidation
• Crackles (rales)
• Mental status changes (especially elderly — may be presenting feature)

"Typical" vs. "Atypical" Presentations

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Diagnosis

Clinical Diagnosis

1. New pulmonary infiltrate on imaging
2. Signs/symptoms of acute respiratory infection (fever, cough, sputum, dyspnea, pleuritic pain)
3. Physical exam findings (crackles, bronchial sounds, dullness)

Chest Imaging

• Chest X-ray: First-line — lobar or segmental consolidation (bacterial), diffuse interstitial infiltrates (atypical/viral), or multilobar infiltrates (severe CAP)
• CT chest: Higher sensitivity, used when CXR equivocal or to evaluate complications (abscess, empyema)

Microbiologic Testing

• Sputum Gram stain and culture: Moderate yield; valuable if good-quality specimen obtained
• Blood cultures (×2): Recommended for hospitalized/severe CAP; positive in ~5–10%
• Urinary antigen tests: S. pneumoniae (sensitivity ~80%), Legionella pneumophila serogroup 1
• PCR / multiplex respiratory panels: Increasingly used for viral and atypical pathogens
• Procalcitonin (PCT): Helps guide antibiotic initiation/duration; low PCT argues against bacterial etiology

Severity Scoring

CURB-65 Score (1 point each):

• Score 0–1: Outpatient treatment
• Score 2: Inpatient observation
• Score ≥3: Hospitalize; consider ICU if score 4–5

Pneumonia Severity Index (PSI / PORT Score)

• More precise tool using 20 variables (age, sex, comorbidities, vitals, labs, radiography)
• Classes I–III → outpatient; Class IV → inpatient; Class V → ICU/intensive care

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Treatment

Site of Care Decision

1. Outpatient: CURB-65 0–1, PSI Class I–II
2. Inpatient (non-ICU): CURB-65 ≥2, PSI Class III–IV
3. ICU: Severe CAP — either 1 major criterion (septic shock requiring vasopressors, mechanical ventilation) or ≥3 minor criteria (RR ≥30, PaO₂/FiO₂ ≤250, multilobar infiltrates, confusion, BUN ≥20 mg/dL, leukopenia, thrombocytopenia, hypothermia, hypotension needing fluids)

Empiric Antibiotic Therapy

Outpatient Treatment

Key update: Newer ATS/IDSA 2019 guidelines allow β-lactam monotherapy in low-risk outpatients (without comorbidities).

Inpatient, Non-ICU (Hemodynamically Stable)

ICU/Severe CAP

Antibiotic Duration

• Outpatient/mild: 5 days (if clinically improving)
• Inpatient: 5–7 days; longer if bacteremic, atypical organisms, or complications
• Use clinical stability criteria (defervescence, improving vitals, tolerating orals) to guide de-escalation

Adjunctive Therapy

• Corticosteroids: Increasingly supported in severe CAP — a 2025 Lancet Respiratory Medicine meta-analysis (PMID: 39892408) identified subgroups most likely to benefit; routine use not universally recommended but may reduce duration/severity in select patients
• IV fluids: Guided by hemodynamic status
• Oxygen therapy: Maintain SpO₂ ≥94% (or ≥88–92% in COPD patients)
• Antipyretics, analgesics for symptomatic relief

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Complications

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Prevention

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Recent Evidence

1. Drug-resistant pathogens in CAP — A systematic review and meta-analysis (PMID: 40107661) identified key risk factors for MDR organisms in CAP, refining who needs broader empirical coverage beyond standard regimens.
2. Empiric therapy for moderate-to-severe CAP — A network meta-analysis in Clinical Microbiology and Infection (PMID: 40602496) compared antibiotic regimens for hospitalized CAP, informing choice of combination vs. monotherapy.
3. Corticosteroids in CAP — A data-driven meta-analysis in Lancet Respiratory Medicine (PMID: 39892408) identified specific patient subgroups likely to benefit from adjuvant corticosteroids, moving toward individualized therapy.

A 2025 ATS/IDSA guideline update has been published (as noted by GuidelineCentral); consult the current ATS/IDSA 2025 guideline for the latest recommendations, as they may update the 2019 framework.

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Key Takeaways

• CAP is defined by community acquisition and diagnosed by new infiltrate + clinical syndrome
• S. pneumoniae remains the leading bacterial cause; viral etiology (including COVID-19) accounts for up to 30%
• Severity scoring (CURB-65, PSI) drives site-of-care decisions
• Empirical therapy is stratified by comorbidities, risk factors, and severity — β-lactam monotherapy is now acceptable in truly low-risk outpatients
• Cardiac complications (AF, ischemia, heart failure) develop in 20–25% of hospitalized patients and are major drivers of morbidity
• Vaccination against pneumococcus, influenza, and COVID-19 is the cornerstone of prevention