Outcomes of early repeat sweat testing in infants with cystic fibrosis transmembrane conductance regulator-related metabolic syndrome/CF screen-positive, inconclusive diagnosis

Background: Reaching early and definitive diagnosis in infants with cystic fibrosis (CF) transmembrane conductance regulator-related metabolic syndrome (CRMS)/CF screen-positive, inconclusive diagnosis (CFSPID) is a priority of all CF newborn screening programs. Currently, sweat testing (ST) is the gold standard for CF diagnosis or exclusion. We assessed outcomes in a cohort of Italian CRMS/CFSPID infants who underwent repeat ST in the 1st year of life. Methods: This multicentre, prospective study analysed clinical data and outcomes in CRMS/CFSPID infants born between September 1, 2018, and December 31, 2019, and followed until June 30, 2020. All subjects underwent CF transmembrane conductance regulator (CFTR) gene sequencing and the search for CFTR macrodeletions/macroduplications, and repeat ST in the 1st year of life. Results: Fifty subjects (median age at end of follow-up, 16 months [range, 7–21 months]) were enrolled. Forty-one (82%) had the first sweat chloride (SC) in the intermediate range. During follow up, 150 STs were performed (range, 1–7/infant). After a median follow-up of 8.5 months (range, 1–16.2 months), 11 (22%) subjects were definitively diagnosed as follows: CF (n = 2 [4%]) at 2 and 5 months, respectively; healthy carrier (n = 8 [16%]), at a median age of 4 months (range, 2–8 months); and healthy (n = 1 [2%]) at 2 months of age. Inconclusive diagnosis remained in 39 (78%) infants. Conclusions: Early repeat ST in the 1st year of life can shorten the time to definitive diagnosis in screening positive subjects with initial SC levels in the intermediate range.