Community-Acquired Pneumonia - CAP


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


          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


  • 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

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.


  • 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


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
  • Physical exam – dullness to percussion, egophony, tachycardia, rales, bronchial breath sounds, tachypnea
  • Complications
    • Respiratory failure
    • Acute respiratory distress syndrome (ARDS)
    • Empyema
    • Sepsis


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
Test Name and Number Recommended Use Limitations Follow Up
CBC with Platelet Count and Automated Differential 0040003
Method: Automated Cell Count/Differential

May be useful in differentiating bacterial from viral infection

Electrolyte Panel 0020410
Method: Quantitative Ion-Selective Electrode/Enzymatic

Evaluate for organ dysfunction

Comprehensive Metabolic Panel 0020408
Method: Quantitative Ion-Selective Electrode/Quantitative Enzymatic/Quantitative Spectrophotometry

Evaluate for organ dysfunction

Respiratory Culture and Gram Stain 0060122
Method: Stain/Culture/Identification

Identify organism of pneumonia

Variable yield because sputum may be hard to obtain

Blood Culture 0060102
Method: Continuous Monitoring Blood Culture/Identification

Utility is questionable

Yield <20%; lower if antibiotic therapy initiated

Limited to the University of Utah Health Sciences Center only

Respiratory Viruses Rapid Culture 2001504
Method: Cell Culture/Immunofluorescence

Identify organism of pneumonia

Respiratory viruses that can be isolated include influenza A & B; parainfluenza types 1, 2, 3; adenovirus; and RSV

Other viruses (eg, HSV or CMV) are not routinely detected

Influenza Virus A and B DFA with Reflex to Influenza Virus A and B Rapid Culture 0060284
Method: Direct Fluorescent Antibody Stain/Cell Culture

Rapid detection of influenza with rapid culture backup

Mycoplasma pneumoniae by PCR 0060256
Method: Qualitative Polymerase Chain Reaction

Determine whether M. pneumoniae is the cause of pneumonia

Streptococcus pneumoniae Antigen, Urine 0060228
Method: Qualitative Immunochromatography

Rapid diagnostic test for invasive disease from S. pneumoniae

False positives may occur because of cross-reactivity with other members of S. mitis group

Clinical correlation is recommended

Patients who have received the S. pneumoniae vaccines may test positive in the 48 hours following vaccination; avoid testing within 5 days of receiving vaccination

Legionella pneumophila Antigen, Urine 0070322
Method: Qualitative Enzyme-Linked Immunosorbent Assay

Rapid diagnostic test with good sensitivity and high specificity

Detects L. pneumophila serogroup 1 antigens

Legionella Species by PCR 0056105
Method: Qualitative Polymerase Chain Reaction

Rapid diagnostic test

Aid in diagnosing etiology of pneumonia when Legionella is suspected

Detect several medically relevant Legionella species, including L. pneumophila, L. micdadei, L. bozemanii, L. longbeachae, L. feeleii and L. dumoffii

Increase chances for discovery of organism in patient partially treated with empirical antibiotics

Very high sensitivity and specificity

Only for respiratory secretions

Negative result does not rule out the presence of PCR inhibitors nor the presence of Legionella organisms in numbers lower than the limit of detection

Chlamydia pneumoniae by PCR 0060715
Method: Qualitative Polymerase Chain Reaction

Confirm C. pneumoniae as infectious agent in nasal wash, nasopharyngeal aspirate, bronchoalveolar lavage (BAL), or pleural fluid

More sensitive than DFA

Body Fluid Culture and Gram Stain 0060108
Method: Stain/Culture/Identification

Positive culture aids in diagnosis of CAP

Procalcitonin 0020763
Method: Immunofluorescence

Use in respiratory tract infections to differentiate need to treat with antibiotics

Additional Tests Available
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
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