Community-Acquired Pneumonia - CAP

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. Laboratory testing includes CBC and metabolic profiles.

  • Diagnosis
  • Background
  • Lab Tests
  • References
  • Related Topics
  • Videos

Indications for Testing

  • Cough, fever, shortness of breath

Laboratory Testing

  • Diagnostic testing other than CBC and metabolic profiles rarely affect therapy for community-acquired pneumonia (CAP) – exception is 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 toxic-appearing patients, patients >55 years
  • Testing recommendations – based on Infectious Disease Society of America (IDSA)/American Thoracic Society (ATS) consensus guidelines, 2007


Culture (optional for outpatients) – useful if therapy would be altered

(Blood = Ba; Sputum = Sb)

UAT (urinary antigen test)

(Legionella = L; Pneumococcal = P)


Intensive care unit admission

B, S

L, P


Failure of outpatient antibiotic therapy


L, P

Cavitary infiltrates

B, S






Active alcohol abuse

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


a Blood culture – positive yield <20%; lower yield if antibiotics started prior to culture.

b 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.

c Endotracheal aspirate if intubated, possibly bronchoscopy or nonbronchoscopic bronchoalveolar lavage

d Fungal and tuberculosis cultures

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

f Special media/culture required for Legionella

g Thoracentesis and pleural fluid cultures

  • 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
      • Use in respiratory tract infections to determine need to treat with antibiotic
      • 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 30-day 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 (BTS)
      • 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
      • CURB-65 + PCT – improved ability of CURB-65 to predict risk of complications
    • Infectious Diseases Society of America/American Thoracic Society consensus guidelines for intensive care unit admission – presence of one major criteria or ≥3 minor criteria
    • 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
      • Criteria for severe community-acquired pneumonia (IDSA/ATS) (calculator)

Differential Diagnosis


  • 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


  • Organism not identified in >35-40% of patients
  • 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 – 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
    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.

    CBC with Platelet Count and Automated Differential 0040003
    Method: Automated Cell Count/Differential

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

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

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


    Variable yield because sputum may be hard to obtain

    Blood Culture 0060102
    Method: Continuous Monitoring Blood Culture/Identification


    Limited to the University of Utah Health Sciences Center only

    Respiratory Viruses Rapid Culture 2001504
    Method: Cell Culture/Immunofluorescence


    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

    Mycoplasma pneumoniae by PCR 0060256
    Method: Qualitative Polymerase Chain Reaction

    Streptococcus pneumoniae Antigen, Urine 0060228
    Method: Qualitative Immunochromatography


    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


    Detects L. pneumophila serogroup 1 antigens

    Legionella Species by Qualitative PCR 2010125
    Method: Qualitative Polymerase Chain Reaction


    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

    Chlamydia pneumoniae by PCR 0060715
    Method: Qualitative Polymerase Chain Reaction

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


    Anaerobe culture is NOT included with this order

    Procalcitonin 0020763
    Method: Immunofluorescence

    Explify Respiratory Pathogens by Next Generation Sequencing 2013694
    Method: Massively Parallel Sequencing


    Bradley JS, Byington CL, Shah SS, Alverson B, Carter ER, Harrison C, Kaplan SL, Mace SE, McCracken GH, Moore MR, St Peter SD, Stockwell JA, Swanson JT. The management of community-acquired pneumonia in infants and children older than 3 months of age: clinical practice guidelines by the Pediatric Infectious Diseases Society and the Infectious Diseases Society of America. Clin Infect Dis. 2011; 53(7): e25-76. PubMed

    Devitt M. PIDS and IDSA issue management guidelines for community-acquired pneumonia in infants and young children. Am Fam Physician. 2012; 86(2): 196-202. PubMed

    Fine MJ, Auble TE, Yealy DM, Hanusa BH, Weissfeld LA, Singer DE, Coley CM, Marrie TJ, Kapoor WN. A prediction rule to identify low-risk patients with community-acquired pneumonia. N Engl J Med. 1997; 336(4): 243-50. PubMed

    Mandell LA, Wunderink RG, Anzueto A, Bartlett JG, Campbell D, Dean NC, Dowell SF, File TM, Musher DM, Niederman MS, Torres A, Whitney CG, Infectious Diseases Society of America, American Thoracic Society. Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults. Clin Infect Dis. 2007; 44 Suppl 2: S27-72. PubMed

    Niederman MS, Luna CM. Community-acquired pneumonia guidelines: a global perspective. Semin Respir Crit Care Med. 2012; 33(3): 298-310. PubMed

    Soni NJ, Samson DJ, Galaydick JL, Vats V, Pitrak DL, Aronson N. Procalcitonin-Guided Antibiotic Therapy. Agency for Healthcare Research and Quality. 2012; 12 (13). PubMed

    General References

    Bartlett JG. Diagnostic tests for agents of community-acquired pneumonia. Clin Infect Dis. 2011; 52 Suppl 4: S296-304. PubMed

    Corcoran JP, Wrightson JM, Belcher E, DeCamp MM, Feller-Kopman D, Rahman NM. Pleural infection: past, present, and future directions. Lancet Respir Med. 2015; 3(7): 563-77. PubMed

    Johansson N, Kalin M, Tiveljung-Lindell A, Giske CG, Hedlund J. Etiology of community-acquired pneumonia: increased microbiological yield with new diagnostic methods. Clin Infect Dis. 2010; 50(2): 202-9. PubMed

    José RJ, Periselneris JN, Brown JS. Community-acquired pneumonia Curr Opin Pulm Med. 2015; 21(3): 212-8. PubMed

    Lim WS, Smith DL, Wise MP, Welham SA, British Thoracic Society. British Thoracic Society community acquired pneumonia guideline and the NICE pneumonia guideline: how they fit together Thorax. 2015; 70(7): 698-700. PubMed

    Musher DM, Thorner AR. Community-acquired pneumonia N Engl J Med. 2014; 371(17): 1619-28. PubMed

    Plouffe JF, Martin DR. Pneumonia in the emergency department. Emerg Med Clin North Am. 2008; 26(2): 389-411, ix. PubMed

    Waterer GW, Rello J, Wunderink RG. Management of community-acquired pneumonia in adults. Am J Respir Crit Care Med. 2011; 183(2): 157-64. PubMed

    Watkins RR, Lemonovich TL. Diagnosis and management of community-acquired pneumonia in adults. Am Fam Physician. 2011; 83(11): 1299-306. PubMed

    References from the ARUP Institute for Clinical and Experimental Pathology®

    Diaz MH, Benitez AJ, Cross KE, Hicks LA, Kutty P, Bramley AM, Chappell JD, Hymas W, Patel A, Qi C, Williams DJ, Arnold SR, Ampofo K, Self WH, Grijalva CG, Anderson EJ, McCullers JA, Pavia AT, Wunderink RG, Edwards KM, Jain S, Winchell JM. Molecular detection and characterization of Mycoplasma pneumoniae among patients hospitalized with community-acquired pneumonia in the United States. Open Forum Infect Dis. 2015; 2(3): ofv106. PubMed

    Diaz MH, Cross KE, Benitez AJ, Hicks LA, Kutty P, Bramley AM, Chappell JD, Hymas W, Patel A, Qi C, Williams DJ, Arnold SR, Ampofo K, Self WH, Grijalva CG, Anderson EJ, McCullers JA, Pavia AT, Wunderink RG, Edwards KM, Jain S, Winchell JM. Identification of bacterial and viral codetections with Mycoplasma pneumoniae using the TaqMan Array Card in patients hospitalized with community-acquired pneumonia. Open Forum Infect Dis. 2016; 3(2): ofw071. PubMed

    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. Community-acquired pneumonia requiring hospitalization among U.S. children N Engl J Med. 2015; 372(9): 835-45. PubMed

    Self WH, Williams DJ, Zhu Y, Ampofo K, Pavia AT, Chappell JD, Hymas WC, Stockmann C, Bramley AM, Schneider E, Erdman D, Finelli L, Jain S, Edwards KM, Grijalva CG. Respiratory Viral Detection in Children and Adults: Comparing Asymptomatic Controls and Patients With Community-Acquired Pneumonia. J Infect Dis. 2016; 213(4): 584-91. PubMed

    Medical Reviewers

    Last Update: November 2017