What finding is indicative of a negative newborn screening for Cystic Fibrosis?

A negative newborn screening for Cystic Fibrosis (CF) is indicated by low immunoreactive trypsinogen (IRT) levels. In newborn screening for CF, an elevated level of IRT is typically the first result that prompts further testing, as it suggests a higher likelihood of the disease. Conversely, low IRT levels are consistent with a negative screening outcome, indicating that CF is unlikely.

The other options represent different clinical conditions. High sweat chloride is indicative of CF, as it reflects the dysfunction of the CFTR gene associated with the disorder. The presence of phenylketonuria (PKU) is unrelated to CF and represents another metabolic disorder, requiring different testing and management. High levels of bilirubin can indicate issues such as jaundice or liver function problems but are not relevant to the screening for CF specifically. Thus, low immunoreactive trypsinogen levels are a clear marker signifying a negative result for Cystic Fibrosis screening.