Community-acquired pneumonia

Definition

Community-acquired pneumonia (CAP) is an acute infection of the lung parenchyma acquired outside hospital. It is distinguished from hospital-acquired (nosocomial) pneumonia and from pneumonia in severe immunodeficiency, which have a different spectrum of pathogens.

Classification

• By X-ray pattern: lobar or multilobar consolidation, interstitial pneumonia, cavitating pneumonia.
• By pathogen: bacterial, "atypical" (chlamydiae, mycoplasmas, legionellae) and viral pneumonia.
• By severity: grading with scores such as CRB-65, CURB-65 and the Pneumonia Severity Index (PSI).

Aetiopathogenesis

• Most common bacterial pathogens: Streptococcus pneumoniae, Hemophilus influenzae, Chlamydia pneumoniae and Mycoplasma pneumoniae. C. pneumoniae is the second most common cause of lung infections in healthy people aged 5 to 35.
• Viruses: coronaviruses (mainly SARS-CoV-2 since 2020), RSV, adenoviruses, influenza, metapneumo- and parainfluenza viruses. Seasonal influenza predisposes to severe secondary bacterial pneumonia.
• Staphylococcus aureus including MRSA: like S. pneumoniae, can cause necrotizing and cavitating pneumonia.
• Pseudomonas aeruginosa: mainly in severe COPD or other structural lung disease, previous Pseudomonas colonization and a hospital stay within the past three months.
• Rare pathogens: Chlamydia psittaci after contact with parrots and related birds, Q fever and tularemia after animal contact.

Even with targeted testing, a pathogen is identified in fewer than 50 % of cases.

Clinical features

• Fever, cough, sputum, pleuritic chest pain, breathlessness
• Tachypnea and tachycardia
• Often nonspecific in older people, e.g. with confusion

Complications: parapneumonic effusion and pleural empyema, necrotizing pneumonia and lung abscess, sepsis, acute respiratory failure.

Diagnosis

The diagnosis is suspected from the clinical picture and confirmed by an infiltrate on chest X-ray. With high clinical suspicion and a normal X-ray, CT or a repeat X-ray may be considered.

• X-ray pattern as a clue: multilobar infiltrates suggest S. pneumoniae or Legionella, an interstitial pattern suggests viruses or mycoplasmas, cavitation suggests S. aureus, fungi or mycobacteria. The X-ray does not reliably identify the pathogen.
• Laboratory tests in moderately severe and severe pneumonia: white cell count, electrolytes, urea and creatinine; pulse oximetry or arterial blood gas analysis to assess oxygenation.
• Microbiology in moderately severe and severe pneumonia: two sets of blood cultures, sputum Gram stain and culture, urinary antigen tests for pneumococci and legionellae, multiplex PCR for respiratory viruses, HIV test. Urinary antigen tests are rapid and more sensitive and specific than sputum Gram stain and culture for these pathogens.

Severity scores

• CRB-65 criteria: C=Confusion + R=Respiratory rate ≥ 30/min + B=BP (sys < 90 or dia ≤ 60) + 65=Age ≥ 65 y.
• CURB-65 additional criterion: urea > 7 mmol/l.
• PSI (Pneumonia Severity Index): more complex score with 20 variables, alternative risk stratification.

Further criteria of severe CAP: hypotension requiring fluids, respiratory rate above 30/min, PaO₂/FiO₂ below 250, multilobar pneumonia, confusion, blood urea nitrogen above 7 mmol/L, white cell count below 4,000/µL, platelets below 100,000/µL and body temperature below 36 °C.

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Further reading (open access)

1. MSD Manual Profi-Ausgabe: Ambulant erworbene Pneumonie
2. AWMF-Leitlinienregister 020-020: Ambulant erworbene Pneumonie
3. StatPearls: Community-Acquired Pneumonia
4. AWMF-Leitlinienregister 020-020: Ambulant erworbene Pneumonie (S3-Leitlinie, Langfassung 2021, PDF)
5. Feldman C, Anderson R: Overview of community-acquired pneumonia (PMC3886318)

Cross-references