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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- CAP requires compatible clinical features plus new infiltrate on chest imaging
- Streptococcus pneumoniae remains the classic typical cause; Mycoplasma, Chlamydia, Legionella, and respiratory viruses are high-yield atypical causes
- Use PSI preferentially or CURB-65 to help decide outpatient vs inpatient care
- Outpatient therapy depends on comorbidities and resistance risk; inpatient non-severe CAP usually requires beta-lactam plus macrolide or respiratory fluoroquinolone
- Severe CAP, hypoxemia, sepsis, empyema, or failure to improve should prompt admission, broader workup, and complication assessment
Overview
Community-acquired pneumonia is infection of the lung parenchyma acquired outside health care settings. The 2019 ATS/IDSA guideline emphasizes radiographic confirmation, severity assessment, empiric therapy based on site of care and risk factors, and avoidance of routine sputum and blood cultures in low-risk outpatients. Classic pathogens include Streptococcus pneumoniae, Haemophilus influenzae, respiratory viruses, Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella species, and Staphylococcus aureus after influenza.
Epidemiology
CAP causes millions of outpatient visits and more than 1 million adult hospitalizations annually in the United States. Incidence and mortality rise sharply with age, chronic cardiopulmonary disease, diabetes, immunosuppression, smoking, alcohol use disorder, and residence in long-term care. Pneumonia remains one of the leading infectious causes of death in the United States, especially among older adults.
Clinical Features
Symptoms
Cough with or without sputum production
Fever, chills, malaise, and pleuritic chest pain
Dyspnea or tachypnea
Confusion or functional decline in older adults
Hemoptysis, severe pleuritic pain, or persistent fever despite therapy
GI symptoms, headache, hyponatremia, or contaminated water exposure suggesting Legionella
Signs
Crackles, bronchial breath sounds, egophony, or dullness to percussion
Tachypnea, tachycardia, fever, or hypoxemia
Sepsis: hypotension, altered mental status, cool extremities, or elevated lactate
Signs of pleural effusion: decreased breath sounds and dullness at the base
Respiratory distress requiring high-flow oxygen or ventilatory support
Investigations
First-line
Chest X-rayNew infiltrate confirms pneumonia in compatible clinical context; lobar consolidation suggests typical bacterial pneumonia
Pulse oximetryAssess severity and need for admission or oxygen therapy
CBC and BMPLeukocytosis, leukopenia, renal function, sodium. Hyponatremia can suggest Legionella but is not specific
Second-line
PSI or CURB-65Severity assessment. CURB-65: confusion, urea/BUN, respiratory rate, blood pressure, age >=65
Blood cultures and sputum cultureNot routine in uncomplicated outpatient CAP. Obtain for severe CAP, MRSA/Pseudomonas risk, prior isolation, or hospitalization with severe disease
Respiratory viral testingInfluenza and SARS-CoV-2 testing when circulating or clinically suspected
Specialist
Urinary antigen testingLegionella and pneumococcal urinary antigens for severe CAP or epidemiologic suspicion
Chest CTUse if complications, obstruction, malignancy, abscess, empyema, or unclear diagnosis is suspected
ThoracentesisIf significant pleural effusion is present to assess for complicated parapneumonic effusion or empyema
1
Outpatient treatment
- Healthy adult without comorbidities or risk factors: amoxicillin, doxycycline, or macrolide only if local pneumococcal resistance is low
- Comorbidities: amoxicillin-clavulanate or cephalosporin plus azithromycin or doxycycline
- Alternative for comorbid outpatient: respiratory fluoroquinolone such as levofloxacin or moxifloxacin when appropriate
- Treat for at least 5 days and until clinically stable
2
Inpatient non-severe CAP
- Beta-lactam such as ceftriaxone, cefotaxime, ampicillin-sulbactam, or ceftaroline plus azithromycin
- Alternative: respiratory fluoroquinolone monotherapy
- Do not routinely use corticosteroids for non-severe CAP
3
Severe CAP or special risks
- Severe CAP: beta-lactam plus azithromycin or beta-lactam plus respiratory fluoroquinolone
- Add MRSA coverage with vancomycin or linezolid if validated risk factors are present
- Add Pseudomonas coverage if prior isolation, recent hospitalization with IV antibiotics, or validated local risk
- Drain empyema or complicated parapneumonic effusion; antibiotics alone are insufficient for frank pus
Complications
- Parapneumonic effusion: Reactive pleural fluid adjacent to pneumonia; complicated effusions require drainage
- Empyema: Pus in the pleural space, low pH, low glucose, or positive Gram stain/culture
- Lung abscess: Necrotizing infection, often aspiration-related, with cavitary lesion and foul sputum
- Sepsis: Hypotension, organ dysfunction, and elevated lactate
- ARDS: Severe inflammatory lung injury with bilateral infiltrates and hypoxemia
USMLE Step 2 CK Exam Tips
- 1Pneumonia requires an infiltrate on chest imaging; bronchitis has cough without infiltrate
- 2Rust-colored sputum and lobar pneumonia = Streptococcus pneumoniae
- 3Atypical pneumonia with bullous myringitis = Mycoplasma pneumoniae
- 4Pneumonia plus diarrhea, confusion, hyponatremia, and water exposure = Legionella
- 5Post-influenza cavitary pneumonia or severe necrotizing pneumonia = Staphylococcus aureus, including MRSA
- 6Recurrent pneumonia in the same anatomic location = obstructing lung cancer until proven otherwise
- 7Empyema needs chest tube drainage; antibiotics alone are not enough
practicetest your knowledge on community-acquired pneumoniaApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — respiratory and beyond.
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