Neonatal cystic fibrosis screening: Analysis and differences in immunoreactive trypsin levels in newborns with a positive screen.

INTRODUCTION: Neonatal cystic fibrosis (CF) screening has enabled the disease to be diagnosed early, and is a determining factor in the increase in survival of these patients. Its main disadvantage is its low specificity and elevated number of false positives. The aim of this study is to analyse the differences in immunoreactive trypsin (IRT) between the different groups of newborns (NB) with a positive neonatal screen depending on whether they were healthy, healthy carriers, affected by CF, or CFSPID (Cystic Fibrosis Screen Positive, Inconclusive Diagnosis). MATERIAL: Retrospective analytical study of the concentrations of IRT in NB with a positive neonatal screen for CF born in a tertiary hospital during an 8-year period. RESULTS: A total of 790 NB with a positive neonatal screen for CF were analysed. Of these 86.3% were term, 53% girls, and 11.8% were admitted. The mean IRT value was 79.16 ng/mL (range 60-367). Significantly higher concentrations of IRT were found in those affected by CF compared to the other groups (P < .001). There were higher levels in large prematures (P = .007) and admitted patients (P = .002). There were no differences as regards gender or season. There was a direct correlation of 64% (P = .001) between IRT and sweat test in those affected by CF and CFSPID. A cut-off value of IRT for the diagnosis of CF was calculated from the ROC curve (76.2 ng/mL (S = 95.7%, Sp = 64.5%). CONCLUSIONS: NB with CF have significantly higher levels of IRT than healthy ones, or carriers and CFSPID. Prematurity and hospital admission may also have an influence. A higher IRT value is associated with a higher level in the sweat test.