Editor,—We read the annotation on the diagnosis of cystic fibrosis with great interest.¹ However, some points about the sweat test that were made are potentially misleading and unfairly denigrate a valuable diagnostic tool.

We were interested in the statement that ‘10% of normal adolescents will have sweat salt concentrations greater than 60 mmol/l’. Although this may be the case for sweat sodium, we do not find this for sweat chloride concentrations, provided a cut off of greater than 70 mmol/l as suggested by the author is used rather than 60 mmol/l as quoted in the seventh paragraph. Sweat sodium concentrations do have a greater age dependency² and may be misleading or confusing if chloride is not measured. In our experience, interpretation of sweat electrolytes does not require particular ‘correction for age’ as the cut off for sweat chloride of >70 mmol/l that we …