A qualitative sweat test, measuring sweat conductivity, represents a screening test for CF. Individuals having positive or borderline results should be followed with a quantitative sweat chloride testing. Screening tests may or may not quantitate the amount of sweat collected and may report a result as positive, negative, or borderline or give an actual concentration of sweat analytes . It is not appropriate to follow up a positive newborn screening test for CF with a sweat screening test. Therefore patients must undergo quantitative chloride testing. The CF Foundation has approved the Wescor Macroduct Sweat Chek™ for screening at clinical sites, such as community hospitals, with the criteria that an individual having a sweat conductivity 50 mmol/L or greater should be referred to an accredited CF care center for a quantitative sweat chloride test.* The CF Foundation accredits centers across the US where patients can receive specialized care for their disease. Some older screening methods using unheated collection cups or direct application chloride electrodes have documented problems with evaporation, making them inappropriate for clinical use. Sweat conductivity is approximately 15 mmol/L higher than sweat chloride concentration due to the presence of unmeasured anions, such as lactate and bicarbonate in the conductivity measurement. Laboratories must be careful to report conductivity using the appropriate conductivity reference intervals.
*Reference: LeGrys VA, Yankaskas JR, Quittell LM, Marshall BC, Mogayzel PJ Jr. Diagnostic sweat testing: the Cystic Fibrosis Foundation guidelines. J Pediatr. 2007 Jul;151(1):85-9.