Sweat testing following newborn screening for cystic fibrosis

Sweat testing remains the "gold standard" for the diagnosis of cystic fibro sis (CF) and is a critical component of newborn screening programs. We retr ospectively reviewed sweat test results reported to a neonatal screening pr ogram for CF with respect to completeness of reported results and the value s recorded for sweat chloride (Cl-) and sodium (Na+) concentrations and the Cl-:Na+ ratio in screened infants. Thirty-nine of 85 Delta F508 homozygous (Delta F508/Delta F508) and 270 of 274 Delta F508 heterozygous (Delta F508/-) infants had sweat tests reported to the screening program. Of those, 30 and 213 sweat test reports, respect ively, were complete, i.e., sweat weight, sweat chloride, and sodium were r eported. Three centers accounted for 37 of 68 (54%) incomplete results, and 4 centers performed 4 or less post-screening sweat tests in the study peri od. There were 6 Delta F508 heterozygous infants with sweat Cl- concentrati ons of 40-60 mmol/L and 4 had CF confirmed by additional genotyping (n = 2) or clinical and repeat sweat CI results (n = 2). Forty-one percent of Delt a F508/-infants with sweat Cl- <40 mmol/L had Cl:Na >1. We conclude that the reporting of incomplete sweat tests is common followin g newborn screening for CF, Infants with sweat Cl- levels of 40-60 mmol/L r equire 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. (C) 2000 Wiley-Liss, Inc.