Sweat chloride testing is considered the gold standard in diagnosing cystic fibrosis. A new study published in Clinical Biochemistry analyzes variables that determine sweat rate and sweat chloride’s analytic and diagnostic capacity across the range of observed sweat rates, and assesses the biologic variability of sweat chloride concentration.
The study concluded that collection of sweat from a person’s forearms is preferred, rather than drawing from the upper legs, especially in young infants. Also, sweat chloride concentrations are not highly dependent on sweat rate, and a change in concentration of sweat chloride exceeding 36% “may be considered a clinically significant response to cystic fibrosis transmembrane receptor targeted therapy,” according to the study. Finally, the researchers found that sweat collections amounting to less than 75 mg provide clinically accurate information.
“This work was inspired by our desire to reduce the number of inadequate sweat collections at our center,” explained Mari L. DeMarco, PhD, lead author of the study and clinical chemist at St Paul’s Hospital and clinical assistant professor at the University of British Columbia in Vancouver. The study, a retrospective analysis, took place at St. Louis Children's Hospital and Washington University in St. Louis.
“The results of this study are valuable to the broader community of medical and laboratory practitioners involved in the care of patients with cystic fibrosis for three reasons,” DeMarco told CLN Stat. The first reason, she explained, is that “characterizing determinants of sweat rate allows for a rational approach to reducing quantity not sufficient rates.” Based on the results of this study, DeMarco said her lab immediately altered its testing protocol, with positive effects. “We now perform bilateral collections on both forearms whenever possible, as opposed to one arm and one leg,” she explained. “This has contributed to a significant drop in quantity not sufficient rates.”
Secondly, “our investigation demonstrated excellent analytic and diagnostic concordance between specimens considered adequate by current criteria and those considered inadequate,” added DeMarco. “Lowering the sweat weight requirement for clinical purposes would obviate the need to discard viable specimens, prevent unnecessary delays in diagnosis, and decrease parental stress associated with repeat testing.” DeMarco said she hopes other testing centers will try to replicate this study “in order to provide additional evidence for a re-examination of the 75 mg cutoff for gauze/paper methods.”
The third reason Marco thinks the study is valuable is that the “estimates of intra-individual variability will allow for the assessment of the significance of a change in sweat chloride concentration in response to therapy,” she explained. “Historically sweat chloride testing has been used in a diagnostic capacity, but with the advent of disease modifying therapies, its role may expand to monitoring response to therapy.”