A normal sweat test is usually taken to measure the concentration of sodium and chloride in the sweat produced. In case of a thermoregulatory sweat test, the focus is mainly on the uniform sweating of an individual based on the raise in temperature above the normal criterion, which is 38 °C and detects the normal functioning of all the sweat glands.
Physiologically, this test consists of giving controlled heat stimulus to an individual under tolerable conditions to induce generalized sweat response in all the areas of skin that are capable of sweating. This test is used to check the functioning of efferent pathways of the nervous system and autonomous, peripheral, as well as central nervous system disorders. It is used to diagnose disorders like small fiber neuropathy, diabetic polyradiculopathy, multiple system atrophy, central system disorders like Parkinson's disease, and pure autonomic failure.
Usually the person undergoing the test is asked to apply any chemical that shows a significant color change when mingled with sweat like corn starch, sodium carbonate, or alizarin red uniformly over the body. This person is then placed in a room with heaters for at least 30 minutes. Once the temperature of the room reaches above the baseline, the subject is seen to sweat profusely and uniformly with characteristic areas showing increased or decreased sweat production. With the color change due to the reaction, these areas can be photographed and could be used for further analysis.
Depending on the intensity of the color change observed, the functioning of the autonomous nervous system would be analyzed. Since this test uses all the sweat glands on the anterior surface of the body, it happens to be the only test to provide us with a widespread geographic screen and distribution of particular nerves.
The accuracy of the results of this test mainly depends on the maximal sweating by the glands. The temperature stimulus would be called as maximal if there is no further sweating after a certain point, which is the thermal load applied over the subject. If the maximal stimulus is not provided, then the chances of the thermoregulatory sweat test showing false positive results are high, when in reality the sweat system is intact. There is no way to find out if the thermal load applied on the patient is maximal or not, so usually the patient has to go through an additional exercise load so that the peak of sweating could be detected easily.
This test is used widely to distinguish Parkinson's disease from multiple system atrophy. The detection of Postural Orthostatic Tachycardia Syndrome (POTS) is also possible by this test. Any disorder that has sweating disabilities as one of their symptoms could be detected by this test, followed by various other tests confirming the presence of that particular syndrome.