The invention describes an in vitro method for detecting a psychiatric disorder occurring with psychosis selected from schizophrenia and bipolar disorder in an individual who has suffered a first psychotic episode, or for determining the state or severity of said disorder, or for monitoring the effect of a therapy administered to an individual who suffers said disorder, or for evaluating the predisposition of an individual presenting a prodromal symptom to develop said disorder, based on the use of the brain-derived neutrophic factor (BDNF) as a marker.