Pneumonia (community acquired)

Essential Evidence

Last Updated on 2021-01-05 © 2021 John Wiley & Sons, Inc.

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Amir H.Barzin, DO, MS, Assistant Professor, University of North Carolina at Chapel Hill
Ryan M. Paulus, DO, Resident Physician , University of North Carolina at Chapel Hill
Stephanie A. Detterline, MD, Assistant Professor, Union Memorial Hospital

Mark H. Ebell, MD, MS, Professor, College of Public Health, University of Georgia
Mindy A. Smith, MD, MS, Clinical Professor, Department of Family Medicine, Michigan State University
Gary Ferenchick, MD, MS, Professor of Medicine, Michigan State University

Overall Bottom Line

  • Consider pneumonia in patients with cough, dyspnea, or sputum production, especially if accompanied by fever, altered breath sounds, or rales. Perform a CXR to confirm the diagnosis. C
  • Determine prognosis using a validated clinical prediction rule such as PSI, CRB-65 (best for primary care) or CURB-65.B
  • Sputum and blood culture, as well as legionella and pneumococcal antigen testing, are recommended for selected patients, particularly those with severe pneumonia and those being treated empirically for MRSA or pseudomonas aeruginosa. C
  • See Table 2 for detailed recommendations regarding antibiotic choice. Treat previously healthy outpatients with amoxicillin, doxycycline, or (if resistance <25%) a macrolide. For outpatients with comorbidities use (amoxiclav or a cephalosporin) plus (macrolide or doxycycline), OR a respiratory fluoroquinolone. Treat for at least 5 days and until the patient has been clinically stable and afebrile for 48 to 72 hours. 103A
  • For inpatients with nonsevere CAP and no risk factors for MRSA or P. aeruginosa, use combination therapy with a beta-lactam plus a macrolide or doxycycline, OR monotherapy with a respiratory fluoroquinolone. For inpatients with severe CAP and no risk factors for MRSA or P. aeruginosa, use a beta-lactam plus macrolide OR beta-lactam plus respiratory fluroquinolone.A
  • For hospitalized patients with severe CAP, consider a systemic corticosteroid, which has been shown to reduce mortality, complications, and length of stay.
  • Assess all patients for vaccine status upon treatment for community-acquired pneumonia (CAP); administer vaccines during outpatient treatment or at hospital discharge, if necessary and available. C


CAP is commonly defined as an acute infection of the pulmonary parenchyma that is associated with at least some symptoms of acute infection, accompanied by the presence of an acute infiltrate on CXR or auscultatory findings consistent with pneumonia, in a non-hospitalized patient or a patient who is diagnosed within 48 hours of hospitalization. 103


  • Pneumonia is present in 3% to 5% of healthy outpatients presenting with acute respiratory illness. 23
  • Pneumonia is diagnosed in 8 to 15/1000 persons per year (4 million adults per year in the United States).

Other Impact

  • Pneumonia is the eighth leading cause of death in the United States. 13
  • Approximately 20% of patients with pneumonia are admitted to the hospital.

Causes of the Condition

  • The most common causes of CAP in outpatients are Streptococcus pneumoniae, staphylococcus aureus, legionella species, Moraxella catarrhalis, Mycoplasma pneumoniae, Haemophilus influenzae, Chlamydia pneumoniae, and respiratory viruses such as influenza A and B, adenovirus, respiratory syncytial virus (RSV), and parainfluenza. 103
  • Among hospitalized patients, causative agents for cases of pneumonia requiring admission in the United States were: mycoplasma (32.5%), viruses (12.7%), S. pneumoniae (12.6%), chlamydia (8.9%), H. influenzae (6.6%), Staphylococcus aureus (3.4%), and legionella (3%). 6
  • A meta-analysis found that among hospitalized patients with CAP (outside of a disease outbreak), 3.5% had chlamydia pneumoniae and 2.7% had legionella. Overall 10% had mycoplasma detected, but this varied considerably by year. 115
  • There is controversy about the role of inhaled corticosteroids (ICS) and pneumonia in patients with COPD. A meta-analysis found that inhaled corticosteroids increase the risk of pneumonia (RR 1.34). 36 Another meta-analysis of inhaled budesonide in patients with COPD did not confirm this finding (rates were 3% for both drug and placebo groups). 45 Finally, an observational study found an increased risk with fluticasone, especially at higher doses, but no increase with budesonide. 57
  • Acid-suppressive drugs increase the risk of CAP: NNH = 449 per year (95% CI, 247–1111) for proton pump inhibitors (PPIs) and 635 for H2 receptor antagonists. 35
  • Malnutrition is an important risk factor for pneumonia incidence and mortality in adults and children.


  • Pneumonia occurs when a particularly virulent organism or large inoculum of a pathogen penetrates the pulmonary defense mechanisms and establishes infection within the lung parenchyma.
  • The defense mechanisms and immune response to an infection can be impaired by many causes, including age, pulmonary disease, anatomic changes, alteration in consciousness, immunosuppressive medications, tobacco use, and other comorbid illnesses.
  • Inflammatory cells are drawn to the infected area of the lung and release proteolytic enzymes, alter the respiratory epithelium and clearance mechanisms, and cause sputum production.

Risk Factors

Risk FactorOdds Ratio
Immunocompromised statusRR 3.1 (1.9–5.1)
Older than 70 yearsRR 1.5 (1.3–1.7)
Alcohol abuseRR 9 (5.1–16.2)
AsthmaRR 4.2 (3.3–5.4)
Lung diseaseRR 3 (2.3–3.9)
Heart diseaseRR 1.9 (1.7–2.3)
InstitutionalizationRR 1.8 (1.4–2.4)
PPI useRR 4.0