Sweat testing was initially standardised in 1959 and it remains the gold standard for diagnosing cystic fibrosis (CF). Guidelines from the US Cystic Fibrosis Foundation and European Cystic Fibrosis Society (ECFS) both note that the sweat test is the preferred method to diagnose CF. Sweat is produced using a technique called iontophoresis, in which an electrical current is used to simulate sweat production and chloride levels are subsequently measured. A sweat chloride level greater than 60 millimoles per litre (mmol/L) is considered diagnostic of CF. In this white paper, Vikki Brandi, Executive Director, Respiratory and Allergy Clinical Development, INC Research considers the performing of sweat testing in a cystic fibrosis clinical trial.