What test result is essential for the diagnosis of cystic fibrosis?

The essential test result for the diagnosis of cystic fibrosis is elevated sweat chloride levels. Cystic fibrosis is a genetic disorder caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, leading to defective chloride channels. The primary diagnostic test measures the concentration of chloride in sweat. Individuals with cystic fibrosis typically have markedly elevated sweat chloride levels, often above 60 millimoles per liter.

This test is crucial because, while other factors such as family history and clinical manifestations (like recurrent respiratory infections) can support the diagnosis, the elevated sweat chloride result provides definitive evidence of the disorder. It reflects the dysfunction of the CFTR protein, which is integral to maintaining proper salt and fluid balance in various epithelial tissues.

Though elevated trypsinogen levels can indicate pancreatic involvement, it is not solely indicative of cystic fibrosis and can occur in other conditions. Therefore, the high sweat chloride level remains the primary criterion for confirming the diagnosis of cystic fibrosis.