Sweating the small stuff: Adequacy and accuracy in sweat chloride determination

Objectives: 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.