Cooperative study comparing three methods of performing sweat tests to diagnose cystic fibrosis.

Directors of cystic fibrosis centers in the United States have noted an increasing number of patients with histories of either false-positive or false-negative sweat tests. These inaccuracies were attributed to the use of rapid test methods which avoided actually weighing the sweat collected. These rapid tests have inherent difficulties which, theoretically at least, could lead to mistaken diagnoses. To evaluate methods of performing the sweat test, the National Cystic Fibrosis Foundation organized a combined study comparing the older Quantitative pilocarpine iontophoretic test (QPIT) method of performing the test with two newer and more rapid methods, the Orion skin electrode, and the Medtherm conductivity apparatus. Five cystic fibrosis centers participated in the study. Although two centers obtained considerably more accurate results with the Orion and the Medtherm than did the other three centers, the combined results of the study indicate that these procedures can be considered to be little more than screening tests.