Palao et al believe that a "clear lower limit for the antipsychotic activity of plasma concentrations of haloperidol" exists and that this limit may be found in our data with appropriate analyses. They support this contention by a χ2 test using our data; our patients with plasma concentrations of haloperidol lower than the alleged lower limit or threshold (ie, 5.5 ng/mL) were less likely to improve than those with plasma concentrations greater than this limit. We computed essentially the same χ2 statistic and obtained a similar result,1(p842) but it did not occur to us that this finding would necessarily imply the existence of a threshold. Why should it? This is a surprising inference.
Palao et al believe that the existence of a threshold is "consistent with the finding that an antipsychotic effect occurs when D2 dopamine receptor occupancy reaches approximately 65% to 75%." However, an