Sweat testing following newborn screening for cystic fibrosis

Sweat testing remains the "gold standard" for the diagnosis of cystic fibrosis (CF) and is a critical component of newborn screening programs. We retrospectively reviewed sweat test results reported to a neonatal screening program for CF with respect to completeness of reported results and the values recorded for sweat chloride (Cl(-)) and sodium (Na(+)) concentrations and the Cl(-):Na(+) ratio in screened infants. Thirty-nine of 85 DeltaF508 homozygous (DeltaF508/DeltaF508) and 270 of 274 DeltaF508 heterozygous (DeltaF508/-) infants had sweat tests reported to the screening program. Of those, 30 and 213 sweat test reports, respectively, were complete, i.e., sweat weight, sweat chloride, and sodium were reported. Three centers accounted for 37 of 68 (54%) incomplete results, and 4 centers performed 4 or less post-screening sweat tests in the study period. There were 6 DeltaF508 heterozygous infants with sweat Cl(-) concentrations of 40-60 mmol/L and 4 had CF confirmed by additional genotyping (n = 2) or clinical and repeat sweat Cl results (n = 2). Forty-one percent of DeltaF508/-infants with sweat Cl(-) <40 mmol/L had Cl:Na >1. We conclude that the reporting of incomplete sweat tests is common following newborn screening for CF. Infants with sweat Cl(-) levels of 40-60 mmol/L require further investigation and review, but they almost certainly have CF. The Cl(-):Na(+) ratio does not appear useful in establishing a diagnosis of CF in infants.