PLAB Pediatrics Practice Exam

The definitive test for cystic fibrosis involves measuring the concentration of chloride in sweat, which is done through the sweat chloride test. In individuals with cystic fibrosis, there is a defect in the CFTR gene that results in abnormally high levels of chloride in the sweat. This phenomenon occurs because the CFTR protein, which helps regulate salt and water transport across cell membranes, is dysfunctional. Therefore, measuring elevated sweat chloride levels is a reliable and definitive way to confirm the diagnosis of cystic fibrosis in newborns.

While the sweat sodium test and genetic screening for CFTR mutations are important tools in the diagnosis and management of cystic fibrosis, they do not replace the sweat chloride test as the definitive diagnostic measure. The immunoreactive trypsin test, often used as a newborn screening test, can indicate potential cystic fibrosis but is not definitive on its own. If elevated, further testing with the sweat chloride test is warranted to confirm the diagnosis. Thus, the sweat chloride test is the standard method used to definitively diagnose cystic fibrosis in newborns.