Pneumonia, Community-Acquired

Key Features

Essentials of Diagnosis

• Fever or hypothermia, tachypnea, cough with or without sputum, dyspnea, chest discomfort, sweats or rigors (or both)

• Bronchial breath sounds or inspiratory crackles on chest auscultation

• Parenchymal opacity on chest radiograph (occasionally not evident at presentation)

• Occurs outside of the hospital or within 48 hours of hospital admission

General Considerations

• The most deadly infectious disease in the United States and the ninth leading cause of death overall

• Risk factors for the development of community-acquired pneumonia (CAP) include

  • Advanced age

  • Alcohol use disorder

  • Tobacco use

  • Comorbid medical conditions, especially asthma or chronic obstructive pulmonary disease (COPD)

  • Immunosuppression

• Prospective studies fail to identify the cause in 30–60% of cases, although bacteria are more commonly identified than viruses

• The most common bacterial pathogens

  • Streptococcus pneumoniae (two-thirds of cases)

  • Haemophilus influenzae

  • Mycoplasma pneumoniae

  • Chlamydophila pneumoniae

  • Staphylococcus aureus

  • Neisseria meningitidis

  • Moraxella catarrhalis

  • Klebsiella pneumoniae

• Common viral causes

  • Coronaviruses (SARS-CoV-2, MERS)

  • Influenza

  • Respiratory syncytial virus

  • Adenovirus

  • Parainfluenza virus

• Assessment of epidemiologic risk factors helps in diagnosing pneumonia due to

  • Chlamydophila psittaci (psittacosis)

  • Coxiella burnetii (Q fever)

  • Francisella tularensis (tularemia)

  • Endemic fungi (Blastomyces, Coccidioides, Histoplasma)

  • Sin Nombre virus (hantavirus pulmonary syndrome)

Clinical Findings

Symptoms and Signs

• Acute or subacute onset of fever, cough with or without sputum, and dyspnea

• Rigors, sweats, chills, pleurisy, chest discomfort, and hemoptysis are common

• Fatigue, anorexia, headache, myalgias, and abdominal pain can be present

• Physical findings include

  • Fever or hypothermia

  • Tachypnea

  • Tachycardia

  • Arterial oxygen desaturation

• Altered breath sounds or rales are common

• Dullness to percussion may be found if lobar consolidation or a parapneumonic effusion is present

Differential Diagnosis

• Bacterial pneumonia

• Viral pneumonia

• Aspiration pneumonia

• Pneumocystis jirovecii pneumonia

• Bronchitis

• Lung abscess

• Tuberculosis

• Pulmonary embolism

• Myocardial infarction

• Sarcoidosis

• Lung neoplasm

• Hypersensitivity pneumonitis

• Bronchiolitis, cryptogenic organizing pneumonia (COP)

Diagnosis

Laboratory Tests

• See Table 9–8

• Sputum Gram stain and culture

  • Neither sensitive nor specific for S pneumonia, the most common cause of CAP

  • Usefulness lies in broadening initial coverage, most commonly to cover

    • S aureus (including community-acquired methicillin-resistant strains)

    • Gram-negative rods (including P aeruginosa and Enterobacteriaceae)

• Urinary antigen assays for Legionella pneumophilia and S pneumoniae

  • At least as sensitive and specific as sputum Gram stain and culture

  • Results are available immediately and are not affected by early initiation of antibiotic therapy

  • Positive tests may allow narrowing of initial antibiotic coverage

  • Indications for urinary antigen assay for Legionella pneumophilia

    • Active alcohol use

    • Travel within 2 weeks

    • Pleural effusion

    • ICU admission

  • Indications for urinary antigen assay for S pneumoniae

    • Leukopenia

    • Asplenia

    • Severe disease

    • Rapid influenza and SARS-CoV-2 testing

  • Have intermediate sensitivity but high specificity, with sensitivity depending upon the method of detection (nucleic ...