title: Sweating the small stuff: adequacy and accuracy in sweat chloride determination.
abstract: OBJECTIVE
  Sweat chloride testing is the gold standard for diagnosis of cystic fibrosis (CF). Our objectives were to: 1) describe variables that determine sweat rate; 2) determine the analytic and diagnostic capacity of sweat chloride analysis across the range of observed sweat rates; and 3) determine the biologic variability of sweat chloride concentration.
  
  
  METHODS
  A retrospective analysis was performed using data from all sweat chloride tests performed at St. Louis Children's Hospital over a 21-month period.
  
  
  RESULTS
  A total of 1397 sweat chloride tests (1155 sufficient [≥75 mg], 242 insufficient [<75 mg]), were performed on 904 individuals. The sweat weight collected from forearms was statistically greater than that collected from legs. There was a negligible correlation between sweat weight and chloride concentration (r=-0.06). The mean individual biologic CV calculated from individuals with two or more sweat collections ≥75 mg was 13.1% (95% CI: 11.3-14.9%; range 0-88%) yielding a reference change value of 36%. Using 60 mmol/L as the diagnostic chloride cutoff, 100% of CF cases were detected whether a minimum sweat weight of 75, 40, or 20 mg was required.
  
  
  CONCLUSIONS
  1) Collection of sweat from forearms is preferable to upper legs, particularly in very young infants; 2) sweat chloride concentrations are not highly dependent upon sweat rate; 3) a change in sweat chloride concentration exceeding 36% may be considered a clinically significant response to cystic fibrosis transmembrane receptor targeted therapy, and 4) sweat collections of less than 75 mg provide clinically accurate information.
authors.affiliation: Department of Pathology and Laboratory Medicine, St. Paul's Hospital, University of British Columbia, Vancouver, Canada. Electronic address: mdmrco@mail.ubc.ca.
authors.forename: Mari L
authors.initials: ML
authors.lastname: DeMarco
authors.affiliation: Department of Pathology and Immunology, Washington University School of Medicine, St Louis, MO, USA; Department of Pediatrics, Washington University School of Medicine, St Louis, MO, USA.
authors.forename: Dennis J
authors.initials: DJ
authors.lastname: Dietzen
authors.affiliation: Department of Pathology and Immunology, Washington University School of Medicine, St Louis, MO, USA; Department of Pediatrics, Washington University School of Medicine, St Louis, MO, USA.
authors.forename: Sarah M
authors.initials: SM
authors.lastname: Brown
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D002712:Chlorides / Q000378:metabolism*; D003550:Cystic Fibrosis / Q000175:diagnosis* / Q000378:metabolism; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D057230:Limit of Detection; D008875:Middle Aged; D012189:Retrospective Studies; D013542:Sweat / Q000378:metabolism*; D055815:Young Adult
chemical_list: D002712:Chlorides
keywords: Biological variability; Cystic fibrosis; Limit of quantitation; Sweat chloride; Sweat rate