Laboratory diagnosis of cystic fibrosis.

The demonstration of abnormally high concentrations of electrolytes in eccrine sweat is still the only practical laboratory procedure available for diagnosis of cystic fibrosis. Properly performed, the sweat test is very reliable, but there are many published reports that all of the methods in current use frequently generate incorrect diagnoses. Analysis of potential for error in sweat test methods shows that of the three essential phases involved, stimulation, collection, and analysis, the major cause of intrinsic inaccuracy occurs in the collection process. In this case the problem is due to condensate formation, which leads to the subsequent analysis of nonrepresentative sweat. Human error is also an important cause of false results and is a direct function of the number of critical manual operations involved in the technic. This review provides a critical examination of sweat test methods, identifying problem areas and suggesting ways to improve procedures in the interests of clinically reliable laboratory data in support of diagnosis.