Pneumonia classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Alejandro Lemor, M.D. [2]

Overview

Pneumonia can be classified in community-acquired pneumonia (CAP), healthcare-associated pneumonia (HCAP), ventilator-associated pneumonia (VAP), and hospital-acquired pneumonia (HAP) according to the setting in which the infection took place. Other classifications include typical and atypical pneumonia, based on the presentation; and bacterial, viral, and fungal, based on the causative pathogen.

Classification

Classification by Setting

Despite having several classification schemes, the most clinically relevant classification relates to the setting in which pneumonia was acquired. The following are the 4 categories are defined by the Infectious Diseases Society of America (IDSA) and American Thoracic Society (ATS):

1. Community-acquired pneumonia (CAP)
2. Hospital-acquired pneumonia (HAP)
   1. Healthcare-associated pneumonia (HCAP)

1. 1. Ventilator-associated pneumonia (VAP)
2. Aspiration pneumonia

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 pneumonias 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	Streptococcus pneumoniae Haemophilus influenzae Staphylococcus aureus Escherichia coli Klebsiella pneumoniae Pseudomonas aeruginosa	Mycoplasma pneumoniae Chlamydophila pneumoniae Legionella pneumophila Influenza Parainfluenza Respiratory syncytial virus (RSV) Adenovirus
Common Findings	Fever, chills Productive cough Pleuritic chest pain Dyspnea Leukocytosis with neutrophilia Chest X-ray with consolidation and air bronchograms.	Gradual onset of fever, without chills Non-productive cough Normal WBC count or mild leukocytosis Myalgia Chest X-ray may be normal or show interstitial infiltrates

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 pneumonias)

Classification of Idiopathic Interstitial Pneumonias

Major idiopathic interstitial pneumonias

• Idiopathic pulmonary fibrosis
• Idiopathic nonspecific interstitial pneumonia
• Respiratory bronchiolitis–interstitial lung disease
• Desquamative interstitial pneumonia
• Cryptogenic organizing pneumonia
• Acute interstitial pneumonia

Rare idiopathic interstitial pneumonias

• Idiopathic lymphoid interstitial pneumonia
• Idiopathic pleuroparenchymal fibroelastosis