title: Classification of hypersensitivity pneumonitis: a hypothesis.
abstract: BACKGROUND
  Regardless of the causative antigen, hypersensitivity pneumonitis (HP) is usually classified as 'acute', 'subacute' or 'chronic'. Considerable confusion still surrounds this classification because there are no widely accepted criteria to distinguish the various stages. The objective of this study was to determine whether the current classification of HP truly reflects categories of patients with distinct clinical features.
  
  
  METHODS
  Data obtained from a large prospective multicenter cohort study (the HP Study) were used to divide a cohort of patients with HP into a limited number of categories (clusters) with maximally differing clinical patterns, without prejudgment. The results of this cluster analysis were compared with the current classification of HP (acute, subacute or chronic).
  
  
  RESULTS
  168 patients were included in the analysis. A 2-cluster solution best fitted the data. Patients in cluster 1 (41 patients) had more recurrent systemic symptoms (chills and body aches) and normal chest radiographs than those in cluster 2 (127 patients) who showed significantly more clubbing, hypoxemia, restrictive patterns on pulmonary function tests and fibrosis on high-resolution computed tomography (HRCT). All p values were <0.0001, using Fisher's exact test. Nodular opacities were seen on HRCT as often in cluster 1 as in cluster 2. There was considerable disagreement between the current classification of HP and the results of our analysis.
  
  
  CONCLUSIONS
  The current classification of acute, subacute and chronic HP is not supported by our analysis. Subacute HP is particularly difficult to define.
authors.affiliation: Centre de Pneumologie, Hôpital Laval, Université Laval, 2725 Chemin Ste-Fo, Québec, Qué. G1V 4G5, Canada. yves.lacasse@med.ulaval.ca
authors.forename: Yves
authors.initials: Y
authors.lastname: Lacasse
authors.forename: Moises
authors.initials: M
authors.lastname: Selman
authors.forename: Ulrich
authors.initials: U
authors.lastname: Costabel
authors.forename: Jean-Charles
authors.initials: JC
authors.lastname: Dalphin
authors.forename: Ferran
authors.initials: F
authors.lastname: Morell
authors.forename: Riitta
authors.initials: R
authors.lastname: Erkinjuntti-Pekkanen
authors.forename: Nestor L
authors.initials: NL
authors.lastname: Mueller
authors.forename: Thomas V
authors.initials: TV
authors.lastname: Colby
authors.forename: Mark
authors.initials: M
authors.lastname: Schuyler
authors.forename: Valérie
authors.initials: V
authors.lastname: Jomphe
authors.forename: Yvon
authors.initials: Y
authors.lastname: Cormier
mesh_terms: D000208:Acute Disease; D000328:Adult; D000542:Alveolitis, Extrinsic Allergic / Q000145:classification* / Q000175:diagnosis; D002908:Chronic Disease; D016000:Cluster Analysis; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged