Pulmonary · PANCE / PANRE

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

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) 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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