Clinical and Laboratory Standards Institute document C34-A3--Sweat Testing: Sample Collection and Quantitative Chloride Analysis; Approved Guideline--Third Edition is a guideline for the performance of the sweat test for the diagnosis of cystic fibrosis. The primary audience includes laboratory and clinical personnel responsible for collecting, analyzing, reporting, and evaluating sweat test results. Sweat stimulation, collection, and the quantitative measurement of sweat chloride are described along with quality assurance and result evaluation.