Which diagnostic test is commonly used to confirm a diagnosis of cystic fibrosis?

The sweat chloride assay is the diagnostic test commonly used to confirm a diagnosis of cystic fibrosis because it directly measures the concentration of chloride in sweat. Cystic fibrosis is caused by mutations in the CFTR gene, which leads to the dysfunction of chloride channels in epithelial cells. As a result, individuals with cystic fibrosis have elevated levels of chloride (and sodium) in their sweat due to impaired reabsorption in sweat glands. The sweat chloride assay is both sensitive and specific, making it a reliable tool for diagnosing cystic fibrosis in children.

In contrast, blood cultures are primarily used to detect bacterial infections, urine analysis is useful for assessing kidney function and detecting metabolic issues, and ultrasound is employed for imaging purposes rather than biochemical testing. Hence, these tests do not provide the specific information needed to diagnose cystic fibrosis, distinguishing the sweat chloride assay as the appropriate choice for confirming the diagnosis.