[Peripheral biological markers in psychiatry: value of epidemiologic studies?].

Until recently, the impossibility to study directly the functioning of the brain has led to the development of various means of assessing peripherally the activity of neurotransmitter systems. However, the study of peripheral biological markers in various psychiatric diseases did not improve dramatically the assessment of diagnosis or prognosis by psychiatrists. The example of the dexamethasone suppression test, supposed to be specifically disturbed in endogenous depression, is striking since numerous recent studies have given similar results in other psychiatric diseases. The negativity of these studies, related to various methodological problems, should bring us to reconsider our strategies. The measurement of peripheral biological markers may be interesting to define predisposition factors or to study the correlates of symptomatic but not syndromic dimensions: epidemiological studies could be a helpful tool to help solving these problems. However, the etiological complexity and the multi-factorial origin of most psychiatric diseases make difficult the discovery of a specific biological marker for a given diagnosis.