Sweat tests in cystic fibrosis [letter].

Dr Smalley and her colleagues rightly stress that the commonest reason for 
abnormally high sweat-electrolyte levels, apart from cystic fibrosis, is faulty technique. 
A further cause of false-positive results, and potentially more important than 
the list of rare disorders usually cited, is atopic dermatitis. Dr 
M. Ampola and I investigated ten children with eczema at the 
Tufts New England Medical Center, Boston, and found higher sodium and/or 
chloride levels in sweat obtained from affected skin than in sweat 
from normal skin in the same individuals. In four of these 
atopic children, levels within the diagnostic range for cystic fibrosis were 
obtained (above 60 mmol/l). 