Sweat test

Question

Can an abnormal sweat test also point towards diseases other than CF?

Answer

Thank you for this interesting question.

When a sweat test is performed, the chloride content of the sweat is determined. An increased concentration of chloride in the sweat (>60 mmol/L) is indeed most typical for cystic fibrosis but can occur with other diseases. In medical text books you can find lists of conditions in which increased sweat chloride values have been reported. The list includes hormonal problems (adrenal, thyroid), severe eczema, severe malnutrition as seen in anorexia nervosa, metabolic disorders such as mucopolysaccharidosis and glycogen storage disease. Also intake of certain medications can influence the results of sweat test. Most of these disorders are very rare and can clinically be easily differentiated from CF. There is limited data however if with these disorders the sweat chloride is very elevated such as in CF or just slightly elevated. Practice learns that it usually concerns results in the gray area (chloride content between 30 and 60 mmol/l).

It is always important to check what type of sweat test was performed and whether it was performed correctly, that is, according to international guidelines. Some types of sweat tests do not explore the chloride concentration in sweat but determine the conductivity of sweat. The conductivity depends on the total content of ions in the sweat collection and the range of ‘normal values’ is higher than for classical sweat test. This conductivity test is a screening tool not a diagnostic test.

It remains important to remember that the diagnosis of cystic fibrosis needs to be made by clinical data or family history plus abnormal result of a specific CF diagnostic test such as a sweat test.

I hope this answer satisfies your interest.

Kind regards,
M. Proesmans

22.03.2010