Community-Acquired Pneumonia (CAP): High-Yield Review for USMLE Step 1 and Step 2 CK (2026)

Community-acquired pneumonia is a staple of USMLE infectious-disease questions, testing severity assessment, the site-of-care decision, and empiric antibiotic selection — all reshaped by the 2019 IDSA/ATS guideline. This review covers CAP the way the boards test it, reflecting current US practice. Follow current guidelines and institutional protocols clinically; this reflects guidance as of mid-2026.

What CAP is

Community-acquired pneumonia is an acute infection of the lung parenchyma acquired outside the hospital. The leading pathogen is Streptococcus pneumoniae, with Haemophilus influenzae, atypicals (Mycoplasma, Chlamydophila, Legionella), respiratory viruses, and — in specific hosts — Staphylococcus aureus and Pseudomonas. A major 2019 change was abandoning the "healthcare-associated pneumonia" (HCAP) category, which had driven unnecessary broad-spectrum use. The mechanism is microbial invasion of the alveoli provoking an inflammatory exudate that impairs gas exchange; risk is higher with age, smoking, COPD, immunosuppression and aspiration. Typical bacterial pathogens cause lobar consolidation, while atypicals and viruses tend to cause a more diffuse, interstitial pattern, and Mycoplasma is a classic cause in younger, otherwise healthy patients.

Presentation

Typical CAP presents with fever, a productive cough, pleuritic chest pain and dyspnea, with focal crackles, bronchial breath sounds and signs of consolidation (dullness to percussion, increased fremitus, egophony). Atypical pathogens may cause a more indolent "walking pneumonia" with a dry cough and prominent extrapulmonary symptoms. Boards link specific exposures to organisms — birds to Chlamydophila psittaci, water systems and air conditioning to Legionella, and a post-influenza presentation to S. aureus. Aspiration pneumonia, favoring dependent lung segments, was historically given broad anaerobic cover, but the 2019 guideline reserves anaerobic coverage for suspected lung abscess or empyema rather than routine aspiration.

Diagnosis

The diagnosis is clinical plus a chest radiograph showing an infiltrate; empiric antibiotics are started regardless of the procalcitonin level. Routine sputum and blood cultures are not needed for outpatients, but are obtained in severe disease or where there are risk factors for resistant organisms. Additional testing in severe disease includes pneumococcal and Legionella urinary antigens, and influenza testing when the virus is circulating, with respiratory cultures to allow later de-escalation. Severity and site of care are then assessed.

Severity and site of care

The boards test risk stratification. Use a validated tool — the Pneumonia Severity Index (PSI) is preferred over CURB-65 — together with clinical judgment to decide on admission. Direct ICU admission is indicated for septic shock requiring vasopressors or respiratory failure requiring mechanical ventilation. Otherwise, severe CAP is defined by meeting one major criterion, or three or more minor criteria (including a respiratory rate of 30 or more, a PaO2/FiO2 ratio below 250, multilobar infiltrates, confusion, uremia, leukopenia, thrombocytopenia, hypothermia, or hypotension needing aggressive fluids). Meeting several minor criteria identifies a patient who benefits from higher-intensity care even without overt shock.

Empiric antibiotic therapy

Treatment is matched to setting and comorbidity:

Outpatient, healthy with no comorbidities: amoxicillin, or doxycycline, or a macrolide — a macrolide only where local pneumococcal resistance is below 25%.
Outpatient with comorbidities (chronic heart, lung, liver or kidney disease, diabetes, alcohol use disorder, malignancy or asplenia): a beta-lactam such as amoxicillin-clavulanate plus a macrolide or doxycycline, or a respiratory fluoroquinolone (levofloxacin or moxifloxacin) alone.
Inpatient, non-severe: a beta-lactam plus a macrolide, or a respiratory fluoroquinolone alone.
Inpatient, severe: a beta-lactam plus either a macrolide or a respiratory fluoroquinolone.

Cover MRSA (vancomycin or linezolid) or Pseudomonas (for example piperacillin-tazobactam or cefepime) only when there are validated local risk factors or prior isolation, not by default. Treat for a minimum of 5 days, continued until the patient is clinically stable.

Complications and follow-up

Complications include a parapneumonic effusion and empyema (which may need drainage), lung abscess, respiratory failure and sepsis. Clinical improvement is usually seen within 48 to 72 hours; failure to improve should prompt a search for a resistant or unusual organism, an obstructing lesion, or a complication such as empyema. Routine follow-up chest imaging is not required for everyone, but is considered in older patients or smokers in whom an underlying malignancy is a concern, and vaccination against pneumococcus and influenza reduces future risk.

High-yield exam points and traps

The HCAP category is gone — do not reflexively choose broad-spectrum MRSA and Pseudomonas coverage without specific risk factors.
The PSI is preferred over CURB-65 for the site-of-care decision, alongside clinical judgment.
Do not use macrolide monotherapy in hospitalized patients, and use it for outpatients only where resistance is below 25%.
Match exposures to organisms — birds to psittaci, water to Legionella, post-influenza to S. aureus.
Empiric antibiotics are started regardless of procalcitonin; procalcitonin should not delay treatment.
Treat for a minimum of 5 days and until clinically stable, not longer by default.
Reserve anaerobic coverage for suspected abscess or empyema, not routine aspiration.
Hypoxia, multilobar disease and confusion are red flags for severe pneumonia that may warrant ICU-level care.

Common questions

What is the most common cause of CAP? Streptococcus pneumoniae, followed by Haemophilus influenzae, atypicals such as Mycoplasma and Legionella, and respiratory viruses; in specific hosts, S. aureus and Pseudomonas become relevant.

Which severity score does IDSA/ATS prefer? The Pneumonia Severity Index (PSI) over CURB-65, used together with clinical judgment to decide on hospitalization; it estimates 30-day mortality risk.

What is the outpatient treatment for healthy adults? Amoxicillin, doxycycline, or a macrolide — with a macrolide used only where local pneumococcal resistance is below 25% — for a minimum of 5 days.

What happened to the HCAP category? It was abandoned in the 2019 IDSA/ATS guideline in favor of using validated risk factors for MRSA and Pseudomonas, rather than blanket broad-spectrum coverage, which reduced unnecessary antibiotic use.

How long should CAP be treated? A minimum of 5 days, continued until the patient is afebrile and clinically stable, rather than a fixed longer course.

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