title: The validity of an empirical delineation of heterogeneity in chronic unexplained fatigue.
abstract: OBJECTIVE
  To validate a latent class structure derived empirically from a clinical data set obtained from persons with chronic medically unexplained fatigue.
  
  
  METHODS
  The strategies utilized in this validation study included: recalculating latent class analysis (LCA) results varying random seeds and the number of initial random starting sets; recalculating LCA results by substituting alternate variables to demonstrate a robust solution; determining the statistical significance of between-class differences on disability, fatigue and demographic measures omitted from the data set used for LCA; cross-classifying class membership using established Centers for Disease Control and Prevention (CDC) research criteria for chronic fatigue syndrome (CFS) to compare the relative proportions of subjects designated CFS, chronic fatigue (not CFS) or healthy controls captured by the latent classes.
  
  
  RESULTS
  Recalculation of results and substitution of variables for low-loading variables demonstrated a robust LCA result. Highly significant between-class differences were confirmed between Class 2 (well) and those interpreted as ill/fatigued. Analysis of between-class differences for the fatigue groups revealed significant differences for all disability and fatigue variables, but with equivalent levels of reported activity and reduction in motivation. Cross-classification against established CDC criteria demonstrated that 89% of subjects constituting Class 2 (well) were indeed nonfatigued controls. A general tendency for grouping CFS cases in the multiple symptomatic classes was noted.
  
  
  CONCLUSIONS
  This study established reasonably good validity for an empirically-derived latent class solution reflecting considerable heterogeneity among subjects with medically unexplained chronic fatigue. This work strengthens the growing understanding of CFS as a heterogeneous entity comprised of several conditions with different underlying pathophysiological mechanisms.
authors.affiliation: Centers for Disease Control and Prevention, Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases, Atlanta, GA 30333, USA. EAslakson@cdc.gov
authors.forename: Eric
authors.initials: E
authors.lastname: Aslakson
authors.forename: Uté
authors.initials: U
authors.lastname: Vollmer-Conna
authors.forename: Peter D
authors.initials: PD
authors.lastname: White
mesh_terms: D000328:Adult; D000368:Aged; D001219:Aspartate Aminotransferases / Q000378:metabolism; D015992:Body Mass Index; D003627:Data Interpretation, Statistical; D003937:Diagnosis, Differential; D004185:Disability Evaluation; D015673:Fatigue Syndrome, Chronic / Q000150:complications / Q000175:diagnosis / Q000503:physiopathology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D025341:Principal Component Analysis; D015203:Reproducibility of Results; D012890:Sleep / Q000502:physiology; D012895:Sleep, REM / Q000502:physiology
chemical_list: D001219:Aspartate Aminotransferases