The relationship of haloperidol concentrations to therapeutic response

Data from pharmacokinetics studies that examined the relationship of haloperidol serum concentrations and therapeutic response in schizophrenic patients were reexamined utilizing the method of logistic regression analysis. A linear rather than a curvilinear relationship was obvious between serum haloperidol concentration and therapeutic response according to both the regression analysis and inspection of the distribution of the data. It was concluded that a serum haloperidol concentration in the range of 9 to 15 ng/ml was associated with a 30% decrease in the total Brief Psychiatric Rating Scale (BPRS) score. Haloperidol concentrations above these appear unlikely in the majority of patients to produce any additional reduction of symptoms. The BPRS psychosis factor finding suggested an analogous finding for serum haloperidol concentrations between 12 and 17 ng/ml. The analyses suggest that the probability of response to haloperidol seems to reach a point of diminishing return at concentrations of approximately 9 to 17 ng/ml. Serum concentrations above this limit do not appear to either decrease or increase the probability of response.