Neuroleptic Plasma Level May Predict Response in Patients Who Meet a Criterion for Improvement

To the Editor.—  We propose a novel approach to the analysis of neuroleptic plasma (or serum) level data in relation to clinical response.Across studies, neuroleptic levels have not consistently predicted clinical response.¹ It has often been questioned whether this is partly due to inclusion of treatment-resistant subjects in studies. In the typical study, either a correlation is computed between plasma level and improvement ratings or groups of patients are categorized globally as "improvers" and "nonimprovers" and then are contrasted for plasma level. If a substantial number of nonresponsive patients have been included in a study, then a true relationship between plasma level and improvement in the rest of the patient sample might easily be missed. A number of methodological standards have evolved to address this problem, including the use of fixed and randomized doses (so that plasma levels of nonresponders will not be preferentially pushed higher, thereby