Pneumonia classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Hamid Qazi, MD, BSc [2], Alejandro Lemor, M.D. [3], Serge Korjian M.D.


Several pneumonia classification schemes have been described. The earliest classification was based on the anatomical distribution of the infectious process observed on autopsy and eventually on medical imaging. Advances in microbiology led to a classification based on etiologic group (bacterial, viral, fungal) despite difficulties often encountered in identifying the etiologic agent. With the advent of antibiotics and the rise in resistance, a classification scheme taking into account the setting in which the pneumonia was acquired was introduced to guide empiric therapy. Pneumonia was classified into community-acquired pneumonia (CAP), healthcare-associated pneumonia (HCAP), ventilator-associated pneumonia (VAP), and hospital-acquired pneumonia (HAP). Despite significant overlap, this classification is essential in selecting appropriate antimicrobial therapy.


Classification by Setting

Despite having several classification schemes, the most clinically relevant classification relates to the setting in which pneumonia was acquired. The following 5 categories are defined by the Infectious Diseases Society of America (IDSA) and American Thoracic Society (ATS):[1][2]

  1. Community-acquired pneumonia (CAP): Pneumonia not acquired in a hospital setting or in a long-term care facility.[3]
  2. Hospital-acquired pneumonia (HAP): Pneumonia that occurs after 48 hours (or more) of hospitalization that was absent on admission.
  3. Healthcare-associated pneumonia (HCAP): Pneumonia in patients hospitalized within 90 days of infection, residents of long-term care facility, patients receiving parenteral antibiotics and chemotherapy within 30 days of infection.
  4. Ventilator-associated pneumonia (VAP): Pneumonia that occurs after 48 hours (or more) of endotracheal intubation.[4]
  5. Aspiration pneumonia: Pneumonia occuring after inhalation of colonized oropharyngeal material.[5]

Classification by Microbiological Agent

Another important clinical and laboratory classification of pneumonia is based on the identification of the causative agent. Although it is of major importance for tailoring therapy, approximately one half of pneumonia do not have an identifiable causative organism. This is the main rationale behind using empirical therapy. The main groups of by causative agent are:

  1. Bacterial pneumonia
  2. Viral pneumonia
  3. Fungal pneumonia
  4. Protozoal pneumonia
  5. Idiopathic interstitial pneumonia (non-infectious)

Classification by Symptoms

Pneumonia can also be classified as typical or atypical pneumonia, depending on the clinical manifestations, chest x-ray findings, and the pathogen that causes the infection.

Typical and atypical pneumonias
Typical Pneumonia Atypical Pneumonia
Common Pathogens
Common Findings

Classification by Anatomic Involvement

Despite being the initial classification scheme developed based on findings on autopsy, the anatomic classification is no longer of major clinical importance. Three major classes are observed:

  1. Lobar pneumonia (involving only one lobe of the lung; mostly observed with Streptoccocus pneumoniae or Klebsiella pneumoniae)
  2. Multilobar pneumonia
  3. Interstitial pneumonia (involves the interstitium rather than airways and alveoli; mostly seen with viral and atypical pneumonia)

Classification of Idiopathic Interstitial Pneumonias[6]

Major idiopathic interstitial pneumonias
Rare idiopathic interstitial pneumonias
  • Idiopathic lymphoid interstitial pneumonia
  • Idiopathic pleuroparenchymal fibroelastosis


  1. Mandell, L. A.; Wunderink, R. G.; Anzueto, A.; Bartlett, J. G.; Campbell, G. D.; Dean, N. C.; Dowell, S. F.; File, T. M.; Musher, D. M.; Niederman, M. S.; Torres, A.; Whitney, C. G. (2007). "Infectious Diseases Society of America/American Thoracic Society Consensus Guidelines on the Management of Community-Acquired Pneumonia in Adults". Clinical Infectious Diseases. 44 (Supplement 2): S27–S72. doi:10.1086/511159. Unknown parameter |iss#= ignored (help)
  2. "Guidelines for the Management of Adults with Hospital-acquired, Ventilator-associated, and Healthcare-associated Pneumonia". American Journal of Respiratory and Critical Care Medicine. 171 (4): 388–416. 2005. doi:10.1164/rccm.200405-644ST. ISSN 1073-449X.
  3. Andrews J, Nadjm B, Gant V, Shetty N (2003). "Community-acquired pneumonia". Curr Opin Pulm Med. 9 (3): 175–80. PMID 12682561.
  4. American Thoracic Society. Infectious Diseases Society of America (2005). "Guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia". Am J Respir Crit Care Med. 171 (4): 388–416. doi:10.1164/rccm.200405-644ST. PMID 15699079.
  5. Marik PE (2001). "Aspiration pneumonitis and aspiration pneumonia". N Engl J Med. 344 (9): 665–71. doi:10.1056/NEJM200103013440908. PMID 11228282.
  6. Travis, William D.; Costabel, Ulrich; Hansell, David M.; King, Talmadge E.; Lynch, David A.; Nicholson, Andrew G.; Ryerson, Christopher J.; Ryu, Jay H.; Selman, Moisés; Wells, Athol U.; Behr, Jurgen; Bouros, Demosthenes; Brown, Kevin K.; Colby, Thomas V.; Collard, Harold R.; Cordeiro, Carlos Robalo; Cottin, Vincent; Crestani, Bruno; Drent, Marjolein; Dudden, Rosalind F.; Egan, Jim; Flaherty, Kevin; Hogaboam, Cory; Inoue, Yoshikazu; Johkoh, Takeshi; Kim, Dong Soon; Kitaichi, Masanori; Loyd, James; Martinez, Fernando J.; Myers, Jeffrey; Protzko, Shandra; Raghu, Ganesh; Richeldi, Luca; Sverzellati, Nicola; Swigris, Jeffrey; Valeyre, Dominique (2013). "An Official American Thoracic Society/European Respiratory Society Statement: Update of the International Multidisciplinary Classification of the Idiopathic Interstitial Pneumonias". American Journal of Respiratory and Critical Care Medicine. 188 (6): 733–748. doi:10.1164/rccm.201308-1483ST. ISSN 1073-449X.