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.
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]
Community-acquired pneumonia (CAP): Pneumonia not acquired in a hospital setting or in a long-term care facility.[3]
Hospital-acquired pneumonia (HAP): Pneumonia that occurs after 48 hours (or more) of hospitalization that was absent on admission.
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.
Ventilator-associated pneumonia (VAP): Pneumonia that occurs after 48 hours (or more) of endotracheal intubation.[4]
Aspiration pneumonia: Pneumonia occuring after inhalation of colonized oropharyngeal material.[5]
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:
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.
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:
↑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)
↑"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. ISSN1073-449X.
↑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. ISSN1073-449X.