Borderline sweat test: Utility and limits of genetic analysis for the diagnosis of cystic fibrosis

ObjectiveThe sweat test remains the gold standard for the diagnosis of Cystic Fibrosis (CF) even despite the availability of molecular analysis of Cystic Fibrosis Transmembrane Conductance Regulator gene (CFTR). We investigated the relationship between CFTR mutation analysis and sweat chloride concentration in a cohort of subjects with borderline sweat test values, in order to identify misdiagnosis of CF.Design and methodsIn the period between March 2006 and February 2008 we performed 773 sweat tests in individuals referred for suspect CF. Ninety-one subjects had chloride values in the border-line range. Clinicians required CFTR gene complete scanning on 66 of them.ResultsThe mean value of sweat chloride in the DNA negative subjects was lower than in those with at least one CFTR mutation. Our data indicate that 39 mEq/l is the best sensitivity trade off for the sweat test with respect to genotype.ConclusionsTo optimise diagnostic accuracy of reference intervals, it may be useful to modify from 30 to 39 mEq/l the threshold for sweat chloride electrolytes.