Community-Acquired Pneumonia - CAP

Community-acquired pneumonia (CAP) is pneumonia contracted outside of a hospital or long-term care facility. CAP is a common disease and a frequent cause of morbidity and mortality worldwide. Laboratory testing includes CBC and metabolic profiles.


Indications for Testing

Cough, fever, shortness of breath, new infiltrate on chest x-ray

Laboratory Testing

  • Diagnostic testing other than CBC and metabolic profiles rarely affects therapy for CAP, except in cases of severe CAP requiring hospitalization
    • Most patients are treated empirically without having etiology identified
    • CBC – leukocytosis with left shift suggests bacterial etiology
    • Complete metabolic and electrolyte profile – use only in patients >55 years or with suspected toxicity
    • Urine antigen tests for Legionella and pneumococcus – available and recommended in select situations (see table below)
    • Nasal screening
      • High specificity and negative predictive value for methicillin-resistant Staphylococcus aureus (MRSA) (Parente, 2018)
      • Consider in antimicrobial decision-making for CAP and healthcare-associated pneumonia (Parente, 2018)
  • Testing recommendations – based on Infectious Disease Society of America (IDSA)/American Thoracic Society (ATS) consensus guidelines, 2007
Indication Culture (Optional for Outpatients): Useful if Therapy Would Be Altered UAT Other
ICU admission

B, S

L, P


Failure of outpatient antibiotic therapy


L, P

Cavitary infiltrates

B, S






Current, excessive  alcohol use

B, S

L, P

Chronic severe liver disease



Severe obstructive/structural lung disease


Asplenia (anatomic or functional)



Recent travel (within past 2 weeks)  



Positive Legionella UAT result



Positive pneumococcal UAT result

B, S


Pleural effusion

B, S

P, L


Notes: Blood culture, positive yield <20%; lower yield if antibiotics started prior to culture. Sputum gram stain and culture, quality sample defined as positive for neutrophils and <10 squamous epithelial cells/low-power field. Variable yield; sputum often difficult to obtain. Special media/culture required for sputum testing for Legionella.

aEndotracheal aspirate if intubated, possibly bronchoscopy or nonbronchoscopic bronchoalveolar lavage

bFungal and tuberculosis cultures

cConsider commonly encountered pathogens (eg, hotel/cruise ship stay in previous 2 weeks, consider Legionella species. Refer to IDSA/ATS Consensus Guidelines, table 8: Epidemiologic conditions and/or risk factors related to specific pathogens in community-acquired pneumonia)

dThoracentesis and pleural fluid cultures

B, blood; ICU, intensive care unit; L, Legionella; P, Pneumococcal; S, sputum; UAT, urinary antigen test

  • Specific testing based on clinical presentation and history (British Thoracic Society [BTS], 2009; ATS/IDSA, 2007)
  • 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
      • Test in respiratory tract infections to determine need to treat with antibiotic
      • PCT >2 ng/mL on first day of admission – high risk of progression to shock/secure sepsis
        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

        Source: Agency for Healthcare Research and Quality (AHRQ), 2012

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
  • Computed tomography (CT) – better imaging approach for small pneumonias; however, cost, radiation exposure, and time contraindicate its use for most patients


  • Severity scoring stratifies patients based on 30-day mortality risk
    • 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, PCT adds little to prognostication
        • In high-risk patients, PCT <0.1 ng/mL suggests better prognosis
    • CURB-65 severity score
      • Clinical prediction rule that has been validated for predicting mortality in CAP
      • CURB-65 – confusion, urea nitrogen, respiratory rate, blood pressure, ≥65 years of age
      • CURB-65 + PCT – improved ability of CURB-65 to predict risk of complications
    • IDSA/ATS consensus guidelines for intensive care unit admission – presence of one major criteria or three or more 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
        • Blood urea nitrogen (BUN) >20 mg/dL
        • Leukopenia (white blood cell [WBC] count <4,000 cells/mm3)
        • Thrombocytopenia (platelets <100,000 cells/mm3)
        • Hypothermia (core temperature <36°C [96.8°F])
        • Hypotension requiring aggressive fluid resuscitation
        • Criteria for severe community-acquired pneumonia (IDSA/ATS)

Differential Diagnosis

  • Lower respiratory tract viral infection (including influenza)
  • Chronic obstructive pulmonary disease (COPD) exacerbation
  • Congestive heart failure
  • Aspiration pneumonitis
  • Cancer
  • Bronchiolitis obliterans organizing pneumonia
  • Pulmonary vasculitis
  • Fibrotic disease
  • Autoimmune connective tissue disease
  • Pulmonary embolus with infarction
  • Sarcoidosis
  • Pulmonary alveolar proteinosis


  • Procalcitonin
    • Use in combination with clinical criteria to guide discontinuation of antibiotics (Kalil, IDSA/ATS, 2016)
  • Antibiotic concentration testing
    • Use laboratory testing to target optimal antibiotic concentrations (refer to Therapeutic Drug Monitoring for testing details)
    • Reduces mortality and length of ICU stay and improves clinical cure rates when used to guide treatment



  • Incidence – >5 million cases annually in the U.S.
    • 12/1,000 in Northern Hemisphere
  • Age – more common in children and elderly patients
    • <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

Risk Factors


Clinical Presentation

  • Patient with normal vital signs (absence of tachypnea, tachycardia, and fever) and a normal physical exam will have pneumonia <5% of the time
  • Nonspecific symptoms – fever, cough, shortness of breath, chest pain, sputum production
  • Atypical organisms tend to cause extrapulmonary disease
  • Physical exam – dullness to percussion, egophony, tachycardia, rales, bronchial breath sounds, tachypnea
  • Complications
    • Respiratory failure
    • Acute respiratory distress syndrome (ARDS)
    • Empyema
    • Sepsis

ARUP Lab Tests

Primary Tests

May be useful in differentiating bacterial from viral infection

Evaluate for organ dysfunction

Evaluate for organ dysfunction

Identify potential bacterial cause of pneumonia

Detect presence of bacteria in blood

Limited to the University of Utah Health Sciences Center only

Low volume will result in decreased recovery of pathogens

AlertThis test has been temporarily inactivated. In compliance with guidance issued by government agencies, respiratory viral cultures are being temporarily discontinued for the safety of laboratory staff during the COVID-19 pandemic. PCR testing is available for a variety of specific viruses; refer to the Laboratory Test Directory.

Detect common respiratory viruses

Offers improved turnaround time over respiratory viral culture

Molecular methods may offer improved sensitivity

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

Other viruses (eg, herpes simplex virus, cytomegalovirus, or hMPV) are not routinely detected

AlertThis test has been temporarily inactivated. In compliance with guidance issued by government agencies, respiratory viral cultures are being temporarily discontinued for the safety of laboratory staff during the COVID-19 pandemic. PCR testing is available for a variety of specific viruses; refer to the Laboratory Test Directory.

Not recommended for inpatients or immunocompromised individuals

Acceptable test for the detection of influenza in outpatients

Detect M. pneumoniae bacteria

Aid in the diagnosis of pneumococcal pneumonia

False positives may occur because of cross-reactivity with other members of Streptococcus 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 vaccination

Provide retrospective evidence of suspected L. pneumophila infection

Detects L. pneumophila serogroup 1 antigens

Negative test result does not rule out Legionella infection due to other serogroups or species of Legionella

Positive result may indicate recent or remote infection with serogroup 1

Detect Legionella species

Only for respiratory secretions

Negative result does not rule out the presence of PCR inhibitors in patient specimen or test-specific nucleic acid in concentrations below the level of detection by this test

Identify Staphylococcus aureus colonization

Detect C. pneumoniae in bronchoalveolar lavage (BAL), nasal wash, nasopharyngeal swab, or pleural fluid

Identify bacteria in normally sterile body fluids

Anaerobe culture is recommended for body fluids, tissue, and deep wound/surgical cultures; refer to anaerobe culture and gram stain

For CSF specimens, order CSF culture and gram stain

For blood specimens, order blood culture or blood culture, AFB and fungal

Anaerobe culture is NOT included with this order

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

Detect respiratory pathogens in patients with pneumonia

Related Tests

Diagnose in hospitalized patients or patients with suspected toxicity

Use in conjunction with BUN testing

Assay interference (negative) may be observed when high concentrations of N-acetylcysteine (NAC) are present

Negative interference has also been reported with NAPQI (an acetaminophen metabolite) but only when concentrations are at or above those expected during acetaminophen overdose

Diagnose in hospitalized patients or patients with suspected toxicity

Gold standard test; detects L. pneumophila and other Legionella species in clinical specimens

Medical Experts



Marc Roger Couturier, PhD, D(ABMM)
Associate Professor of Clinical Pathology, University of Utah
Medical Director, Parasitology/Fecal Testing, Infectious Disease Antigen Testing, Bacteriology, and Molecular Amplified Detection, ARUP Laboratories


Mark A. Fisher, PhD, D(ABMM)
Associate Professor of Clinical Pathology, University of Utah
Medical Director, Bacteriology, Special Microbiology, and Antimicrobial Susceptibility Testing, ARUP Laboratories


Additional Resources
Resources from the ARUP Institute for Clinical and Experimental Pathology®
  • 25714161

    Jain S

    Williams DJ

    Arnold SR

    Ampofo K

    Bramley AM

    Reed C

    Stockmann C

    Anderson EJ

    Grijalva CG

    Self WH

    Zhu Y

    Patel A

    Hymas W

    Chappell JD

    Kaufman RA

    Kan H

    Dansie D

    Lenny N

    Hillyard DR

    Haynes LM

    Levine M

    Lindstrom S

    Winchell JM

    Katz JM

    Erdman D

    Schneider E

    Hicks LA

    Wunderink RG

    Edwards KM

    Pavia AT

    McCullers JA

    Finelli L

    CDC EPIC Study Team

    N Engl J Med