Thermoregulatory sweat test

A thermoregulatory sweat test is a specialized test used to diagnose several neurological conditions as well as conditions that affect the sweat glands.
It is performed in a purpose built room that can be precisely controlled for temperature, humidity and air flow. The patient is covered in a water absorbing powder that changes color to purple when exposed to sweat, but remains yellow when dry. The patient is put through a protocol and the results of the color change are both observed by the physician performing the test and are also recorded for later review. Both the timing and pattern of color change are used to determine an appropriate diagnosis. 
Thermoregulatory sweat test at Stanford Medical