A simple test that measures the sweat gland activity of a depressed person can determine if he or she is suicidal - with 97 per cent accuracy - scientists say.
German and Swedish researchers found blood pressure, blood circulation and activity in the sweat glands of the fingers can reveal if a person is suicidal.
"The results are so strong that I'm astonished. We can determine very accurately whether a person risks committing suicide, which can revolutionise suicide prevention," said Lars-Hakan Thorell, associate professor in experimental psychiatry at Linkoping University in Sweden, one of the researchers behind the study.
In the study, 783 depressed in-patients in Germany were tested for hyporeactivity - reduced ability to react to various stimuli.
A suicidal depressed person reacts differently to environmental changes, compared to a healthy person.
The result confirms previous research stating that there is a strong correlation between hyporeactivity and suicide in depressed people.
The test found that hyporeactivity was present in up to 97 per cent of depressed patients who later committed suicide, compared to just 2 per cent of the depressed patients who were not hyporeactive.
The study also found no relation between the severity of depression and hyporeactivity.
"It indicates a certain per cent, even if the normal population can have this neurophysical disorder. Everyone who has it is not suicidal - but almost all suicidal, depressed patients have it," Thorell said.
Hyporeactivity was most prevalent in the bipolar patients: of 126 patients, 80.2 per cent were affected, compared to 67.3 per cent of the depressed patients and 58.5 per cent of those with other diagnoses.
The study also shows that people with recurrent depression run a risk of becoming hyporeactive at some later point in life.
Hyporeactivity can be measured by the test person listening to a pattern of tones, while the body's reactions are measured via sensors on the fingers. The first time they hear a tone, virtually all people react. This is a general orientation reaction which occurs automatically. But when the tone is heard again, the reaction decreases among some people: the hyporeactive.
"A depressed person has a biological inability to care about the surroundings, while a healthy person continues to react," Thorell said.
The study was published in the Journal of Psychiatric Research.
