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For further information, see CMDT Part 9-11: Pneumonia

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)


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 ...

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