Lars-Hakan Thorell, associate professor in experimental psychiatry at Linkoping University, one of the researchers behind the study, said that blood pressure, blood circulation and activity in the sweat glands of the fingers can reveal if a person is suicidal.
In the German-Swedish 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 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.
But the study also shows there is no relation between the severity of depression and hyporeactivity.
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 amongst some people: the hyporeactive.
The study has been published in the Journal of Psychiatric Research.