Cystic fibrosis. Problems encountered with sweat testing

The sweat test is the only practical and reliable laboratory test for confirmation of the diagnosis of cystic fibrosis. Among 234 patients referred to The Johns Hopkins Hospital for sweat testing, 62 had had a previous test; 29 tests had been reported as negative, and 33 tests had been reported as positive. Results of quantitative pilocarpine-iontophoresis sweat tests at this center led to a change in diagnosis in 27 (43.5%) of these 62 patients. Most of the errors were false-positive.