Sweat tests in patients with diabetes insipidus.

Increased sweat chloride concentrations have been reported in patients with nephrogenic (NDI) but not central diabetes insipidus (CDI). To determine whether false-positive sweat tests also occur in CDI, six subjects with CDI had plasma electrolytes, osmolatities, and sweat tests performed before and after water deprivation. All subjects were hyperosmolar after dehydration (287 +/- 2.0 and 296 +/- 3.2 mosm/kg, plasma osmolality before and after dehydration, respectively; p = 0.02). Sweat chloride concentrations before and after dehydration were not different, and no positive sweat tests were observed. Sweat chloride concentrations after dehydration did not correlate with the degree of dehydration as assessed by either plasma sodium or plasma osmolality. We conclude that increased concentrations of chloride in sweat are not typically found in dehydrated subjects with CDI, although they have been reported in subjects with the nephrogenic form.