A simple measurement of the sweat gland activity of a depressed person can determine if he or she is suicidal - with 97 per cent accuracy. Now another large clinical study confirms the correlation.

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", says Lars-Håkan Thorell, associate professor in experimental psychiatry at Linköping University, one of the researchers behind the study. "We can determine very accurately whether a person risks committing suicide, which can revolutionise suicide prevention."

In the German-Swedish study, published in the Journal of Psychiatric Research, 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.

But the study also shows there is no relation between the severity of depression and hyporeactivity. Associate Professor Thorell: "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."

Hyporeactivity was most prevalent in the bipolar patients: of 126 patients, 80.2% were affected, compared to 67.3% of the depressed patients and 58.5% 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.

"It was probably the case that certain nerve cells in the hippocampus are damaged by depressions and negative stress."

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.

"A depressed person has a biological inability to care about the surroundings, while a healthy person continues to react."

Associate Professor Thorell will now engage the spin-off company Emotra to conduct a study in some 15 countries together with his co-authors, including Professor Manfred Wolfersdorf, professor of psychiatry and director of the Bayreuth city clinic, and Professor Wolfgang Kaschka, University Hospital Ulm.