Community-Acquired Pneumonia - CAP

Diagnosis

Indications for testing

• Cough, fever, shortness of breath

Laboratory Testing

• CBC – leukocytosis with left shift suggests bacterial etiology
• Complete metabolic and electrolyte profile – use only in toxic-appearing patients, patients >55 years
• Cultures (IDSA/ATS guidelines) – culture only if therapy would be altered
  • Sputum gram stain and culture – quality sample defined as positive for neutrophils and <25 epithelial cells/high-powered field
    • Variable yield – sputum often difficult to obtain
  • Pleural fluid – thoracentesis if >5 cm height on chest x-ray noted
  • Bronchoscopic lavage for patients who are severely ill and from whom sputum cannot be obtained
  • Blood cultures – consider in patients who will be hospitalized
    • Positive yield <20%
      • Lower yield if antibiotics started prior to culture
• Specific testing (based on clinical presentation and history)
  • Streptococcus pneumoniae – rapid urine antigen testing (consider in alcoholics, asplenia, ICU admissions, chronic illness, leukopenia, pleural effusion)
  • Legionella spp – rapid urine antigen testing (consider in alcoholics, ICU admissions, pleural effusion)
  • Influenza – point-of-care testing or rapid DFA (consider during appropriate season)
  • Mycoplasma pneumoniae – PCR, antibody testing
  • Chlamydophila pneumoniae – PCR, antibody testing
• Differentiation of lower respiratory tract viral infection from bacterial infection
  • Procalcitonin (PCT)
    • Precursor to calcitonin
    • Final step in synthesis is inhibited by endotoxin and cytokines
    • Elevated in bacterial infections
    • Use in respiratory tract infections to determine need to treat with antibiotic
      • Treatment recommendation

        PCT	Treatment Recommendation
        <0.1 ng/mL	Antibiotics strongly discouraged
        0.1-0.24 ng/mL	Antibiotics discouraged
        0.25-0.5 ng/mL	Antibiotics encouraged
        >0.5 ng/mL	Antibiotics strongly encouraged

    • PCT >2ng/mL on first day of admission – high risk of progression to shock/secure sepsis

Imaging Studies

• Chest x-ray – gold standard for confirmation of pneumonia
  • Single or several lobe patterns – bacterial
  • Diffuse or interstitial pattern – viral or atypical organism
  • Cavitary – more common in gram negative, fungi, acid-fast bacilli
  • Miliary – acid-fast bacilli, fungi, atypical pneumonia agents
• CT – better tool for small pneumonias; however, cost, radiation exposure and time negate its use for most patients

Prognosis

• Severity scoring stratifies patients based on mortality
  • Pneumonia severity index (PSI) (based on study by Patient Outcomes Research Team)
    • PSI uses several clinical variables to calculate a score to predict risk of death
    • Classes I-III have low risk of death and can probably be treated as outpatients
    • Classes IV and V have higher risk of death and should probably be treated as inpatients
    • PSI + PCT
      • In low-risk patients, adds little to prognostication
      • In high-risk patients, PCT <0.1ng/mL suggests better prognosis
  • CURB-65 severity score (British Thoracic Society)
    • Clinical prediction rule that has been validated for predicting mortality in community-acquired pneumonia
    • CURB-65 = confusion, urea nitrogen, respiratory rate, blood pressure, ≥65 years of age
  • Criteria for severe community-acquired pneumonia (IDSA/ATS)
  • Criteria for ICU admission – presence of one major criteria or ≥3 minor criteria
    • Major criteria
      • Invasive mechanical ventilation
      • Septic shock with need for vasopressors
    • Minor criteria
      • Respiratory rate >30 breaths/minute
      • Partial pressure of oxygen in arterial blood (PaO₂)/fraction of inspired oxygen (FiO₂) <250
      • New onset of confusion
      • Multilobar infiltrates
      • BUN >20 mg/dL
      • Leukopenia (WBC count <4,000 cells/mm³)
      • Thrombocytopenia (platelets <100,000 cells/mm³)
      • Hypothermia (core temperature <36°C)
      • Hypotension requiring aggressive fluid resuscitation

Differential Diagnosis

• Lower respiratory tract viral infection
• Chronic obstructive disease (COPD) exacerbation
• Cancer
  • Metastases
  • Primary pulmonary tumor
• Bronchiolitis obliterans organizing pneumonia
• Heart failure
• Autoimmune connective tissue disease
• Fibrotic disease
• Vasculitis

Clinical Background

Community-acquired pneumonia (CAP) is pneumonia acquired outside of a hospital or long-term care facility. CAP is a common disease and a frequent cause of morbidity and mortality worldwide.

Epidemiology

• Incidence – >5 million cases annually in the U.S.
  • 12/1,000 in Northern Hemisphere
    • <1 year – 30-50/1,000
    • 15-45 years – 1-5/1,000
    • 60-70 years – 10-20/1,000
    • 71-85 years – 50/1,000
• Age – more common in younger and older patients

Risk Factors

• Older age
• Alcohol abuse
• Asthma
• Chronic preexisting disease – heart, lung, liver
• Immunocompromised state
• Institutionalized patients
• Tobacco abuse

Organisms

• Organism not identified in >35-40% of patients
• Viral
  • Orthomyxoviridae (influenza A and B)
  • Paramyxoviridae
    • Metapneumovirus (hMPV)
    • Parainfluenza virus 1, 2, 3
    • Respiratory syncytial virus (RSV)
  • Coronaviriniae (coronavirus)
  • Adenoviridae (adenovirus)
  • Bunyaviridae (hantavirus)
• Bacterial
  • Streptococcus pneumoniae
  • Haemophilus influenzae
  • Atypical organisms
    • Mycoplasma pneumoniae
    • Legionella spp
    • Chlamydophila pneumoniae
    • Francisella tularensis
    • Coxiella burnetii
  • Staphylococcus aureus
  • Moraxella catarrhalis
  • Gram-negative bacilli
  • Mycobacteria
• Fungi
  • Blastomycosis
  • Coccidioidomycosis
  • Histoplasmosis

Clinical Presentation

• Patient with normal vital signs and a normal physical exam will have pneumonia <5% of the time
• Nonspecific – fever, cough, shortness of breath, chest pain, sputum production
• Atypical organisms tend to cause extra-pulmonary disease
  • Central nervous system – headache, meningitis, encephalopathy
  • Gastrointestinal  – diarrhea, hepatitis
  • Electrolyte abnormalities
  • Renal – renal insufficiency
• Physical exam – dullness to percussion, egophony, tachycardia, rales, bronchial breath sounds, tachypnea
• Complications
  • Respiratory failure
  • Acute respiratory distress syndrome (ARDS)
  • Empyema
  • Sepsis

Treatment

• Rapid initiation of antibiotics in bacterial cases is associated with better outcomes; pathogen-specific therapy advised
  • Refer to Infectious Diseases Society of America/American Thoracic Society consensus guidelines

Indications for Laboratory Testing

• Tests generally appear in the order most useful for common clinical situations
• Click on number for test-specific information in the ARUP Laboratory Test Directory

Click the plus sign to expand the table of additional tests.

Test Name and Number	Comments
Urea Nitrogen, Serum or Plasma 0020023 Method: Quantitative Spectrophotometry	Diagnoses in hospitalized patients or toxic-appearing patients
Creatinine, Serum or Plasma 0020025 Method: Quantitative Enzymatic	Use in conjunction with BUN testing
Glucose, Plasma or Serum 0020024 Method: Quantitative Enzymatic	Diagnoses in hospitalized patients or toxic-appearing patients
Legionella Species, Culture 0060113 Method: Culture/Identification	Determine whether Legionella is the cause of pneumonia; may help in determining treatment and severity