Why does cystic fibrosis cause a person's sweat to be abnormally salty?

Extra loss: Cystic fibrosis is caused by a genetic mutation of cftr protein that moves chloride ions out of cells in respiratory tract, but into the cells in sweat gland ducts. Since cftr protein is not functional in CF pts, excess chloride gets secreted in their sweat, which causes the sweat to have higher salt concentration. That is why a sweat test measuring chloride is used as a diagnostic test for cf.