Biomarkers in the blood can be used to identify people at risk of suicide, a study suggests.
The US research raises the controversial prospect of a blood test providing early warning of suicide attempts. Scientists said their results were "proof of principle" for a suicide test, while British experts reacted with scepticism.
One particular molecule, an enzyme called SAT1, was linked to suicidal tendencies in a group of patients with bipolar disorder, a mental illness characterised by episodes of extreme high and low moods. Three other markers showed a weaker association.
The elevated blood markers stood out in a subgroup of nine patients who displayed a sudden dramatic shift to powerful suicidal thoughts. A similar pattern was seen in blood samples taken from nine suicide victims who had succeeded in killing themselves.
Finally blood samples were tested from two further groups of patients with bipolar disorder and schizophrenia-type psychoses. Raised levels of the biomarkers correlated with admissions to hospital after suicide attempts. The link was stronger for bipolar disorder than for schizophrenia.
Study leader Dr Alexander Niculescu, from Indiana University, said: "Suicide is a big problem in psychiatry. It's a big problem in the civilian realm, it's a big problem in the military realm and there are no objective markers.
"There are people who will not reveal they are having suicidal thoughts when you ask them, who then commit it and there's nothing you can do about it. We need better ways to identify, intervene and prevent these tragic cases. These seem to be good markers for suicidal behaviour in males who have bipolar mood disorders or males in the general population who commit impulsive violent suicide. In the future we want to study and assemble clinical and socio-demographic risk factors, along with our blood tests, to increase our ability to predict risk."
He acknowledged one limitation of the research, published in the journal Molecular Psychiatry, was that the participants were all male. "There could be gender differences," he said. "We would also like to conduct more extensive, normative studies in the population at large."
In their paper, the scientists point to a link between SAT1 and polyamine, a chemical involved in apoptosis, or "cell suicide" - the programmed self destruction of damaged or harmful cells. They wrote: "It could be that.. mechanisms related to cellular survival have been recruited by evolution for higher mental functions, such as feelings, thoughts, actions and behaviours, leading to suicidality. In that sense, suicidality could be viewed as a whole-organism apoptosis."
British scientists said the research should be treated with caution. Professor Keith Hawton, director of the Centre for Suicide Research at Oxford University, said: "There is a big difference between finding differences between groups (as in this study) compared with risk in actual individuals, the latter being the real test of predictors. I would say that the findings are of interest and may point the way to some future research based on large samples, but no more than that."