Just read Psych Central paper on serotonin and depression. Have read 17 other papers, blog posts or articles on "low serotonin". None of them define "low". Can find lab reference ranges from 50 to 310 ng/ml for serum serotonin. Doesn't tell me what is clinically"low". Can some knowledgable person define "low" for clinically low plasma serotonin?