Outcomes of Repeat Sweat Testing in Cystic Fibrosis Newborn Screen Positive Infants.

BACKGROUND Infants with a positive cystic fibrosis (CF) newborn screen, only one identified CFTR mutation (NBS+/1 mut), and an initial intermediate sweat chloride (30-59 mmol/L) should have repeat sweat chloride testing (SCT). However, the outcome of repeat SCT and the relationship between initial sweat Cl and subsequent CF diagnosis have not been reported. OBJECTIVE The objective of this study was to analyze the outcomes of repeat SCT and subsequent CF diagnosis in NBS+/1 mut infants based on their initial sweat chloride concentration. METHODS We retrospectively identified all infants born in Indiana from 2007 to 2017 with NBS+/1 mut and initial SCT in the intermediate range. For each infant, we recorded the initial and repeat SCT results and/or a final CF diagnosis. RESULTS From 2007 through 2017 there were 2822 NBS+/1 mut infants of which 2613 (82%) had at least one SCT result. No infants with an initial SCT of 30-39 mmol/L were subsequently diagnosed with CF. Of the 31 infants with an initial SCT of 40-49 mmol/L, only 1 was subsequently diagnosed with CF. In contrast, 61% of those with SCTs of 50-59 mmol/L were later diagnosed with CF. CONCLUSION These results suggest that infants with a positive NBS for CF and one CFTR mutationwhose initial sweat chloride concentration is 50-59 mmol/L need to be monitored more closely forCF with strong consideration for earlier repeat SCTs and immediate genotyping.