Pulmonary · PANCE / PANRE

Community-Acquired Pneumonia (CAP)

Written & clinically reviewed by Ryan Busch, PA-C · Updated August 2026 · Report an error

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?
  • AAzithromycin
  • BTrimethoprim-sulfamethoxazole (TMP-SMX)
  • CAmoxicillin
  • DCephalexin
Reveal answer & full explanation
Correct answer: C — Amoxicillin
  • AAzithromycin
  • BTrimethoprim-sulfamethoxazole (TMP-SMX)
  • CAmoxicillin✓
  • DCephalexin

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.
Question 2PulmonaryEasy
A 24-year-old college student presents with low-grade fever, dry cough, and headache for 10 days. CXR shows patchy interstitial infiltrates that appear worse than her clinical exam suggests. Cold agglutinins are positive. Which organism is most likely responsible?
  • AStreptococcus pneumoniae
  • BLegionella pneumophila
  • CMycoplasma pneumoniae
  • DPneumocystis jirovecii
Reveal answer & full explanation
Correct answer: C — Mycoplasma pneumoniae
  • AStreptococcus pneumoniae
  • BLegionella pneumophila
  • CMycoplasma pneumoniae✓
  • DPneumocystis jirovecii

Why Mycoplasma pneumoniae is correct

  • Mycoplasma pneumoniae causes 'walking pneumonia' in young adults, presenting with a subacute course, dry cough, headache, and low fever
  • CXR appears worse than the clinical exam suggests, which is the hallmark of 'atypical' pneumonia
  • Cold agglutinins (IgM autoantibodies against erythrocyte I antigen) are present in ~50% of cases and can occasionally cause hemolytic anemia

Why the others are wrong

  • Streptococcus pneumoniae — causes lobar pneumonia with rust-colored sputum
  • Legionella pneumophila — involves GI symptoms, hyponatremia, and water source exposure
  • Pneumocystis jirovecii — occurs in immunocompromised hosts (HIV with CD4 <200)
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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 Score30-Day MortalityDisposition
0-1<3%Outpatient
2~9%Short inpatient stay or supervised outpatient
3-515-40%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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Commonly confused with

vs Influenzavs Acute Bronchitisvs Aspiration Pneumonia and Pneumonitis

Related Pulmonary diagnoses

AsthmaChronic Obstructive Pulmonary Disease (COPD)Hospital-Acquired and Ventilator-Associated Pneumonia (HAP/VAP)Aspiration Pneumonia and PneumonitisTuberculosis (Active and Latent)Pulmonary Embolism (PE)Pulmonary HypertensionPneumothorax (Spontaneous and Tension)Pleural EffusionAcute Respiratory Distress Syndrome (ARDS)Lung Cancer (Small Cell and Non-Small Cell)BronchiectasisCystic FibrosisSarcoidosis

Differentials & related conditions

Acute Bronchitis

Educational use only. This outline is a study aid for PA students and is not medical advice or a substitute for clinical judgment. FirstPassPA is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCCPA or PAEA. PANCE® and PANRE® are registered trademarks of the National Commission on Certification of Physician Assistants; End of Rotation™ is a program of the Physician Assistant Education Association.