A-332 Sweat rate does not influence sweat chloride concentration

Abstract

Abstract Introduction The recommendations for sweat chloride ([Cl-]) testing from the Clinical and Laboratory Standards Institute (CLSI C34-A4 2019) endorse a minimum sweat rate of 1g/m2/min to validate efficient sweat stimulation. For labs that use macroduct ® collection devices, this rate corresponds to a sweat volume ≥15 µL collected within a 30-minute period. Bilateral collection and testing is also advised to improve the odds of obtaining a sufficient specimen from at least 1 site. When both sites fail to meet the minimum rate, the test is invalid (QNS) and should be repeated, potentially delaying Cystic Fibrosis (CF) diagnosis. In this study, we evaluate the premise for the minimum sweat rate by investigating the relationship between sweat rate and [Cl-]. In a sub-analysis, residual sweat available after clinical testing is used to examine the effect of specimen pooling on [Cl-]. This study has the potential to inform best practices for future sweat testing guidelines, decrease QNS rates, and reduce time to CF diagnosis. Methods Thirteen months of data from sweat tests performed at St. Louis Children's Hospital were reviewed. All collections were performed using macroduct devices following CLSI 34-A4 guidelines (results from QNS specimens were not reported clinically). Pearson correlation was calculated to examine the relationship between sweat rate and [Cl-] (n = 354). Deming regression was applied to evaluate the effect of specimen pooling, versus results from individual sites, in 19 collections with residual volume after clinical testing. Results Figure 1 Conclusion Negligible correlation between rate and [Cl-] across a wide range of values suggests the current minimum volume for macroduct collections (15 µL) may be overly stringent. Reporting of [Cl-] in specimens with ≥10 µL sweat may decrease QNS rates without compromising accuracy. Preliminary data suggests pooling of bilateral collections may be a feasible option to achieve the minimum volume required for testing (10 µL).