Use of Plasma Levels of Antipsychotic Drugs
The use of plasma levels as a guide for dosing or determining lack of response to an antipsychotic medication is relatively com-mon in clinical settings, yet it remains controversial (Marder, 1997). There is a wide interindividual difference in blood lev-els in patients on the same dose of an antipsychotic drug, and a narrow dose range between therapeutic efficacy and increasing risk of adverse effects (Van Putten et al., 1991; Kane and Marder, 1993; Marder, 1997). Although plasma levels among first-genera-tion antipsychotics have been established for several compounds, there is at best a moderate correlation between these levels and clinical effects (Kane and Marder, 1993).
It may be useful to monitor the plasma level of an antipsychotic medication under certain circumstances (Fleischhacker, 2001). For example, before deciding that the agent is ineffective despite an adequate trial at a sufficient dose, it is important to determine whether it may be due to alterations in the pharmacokinetics of the drug or nonadherence to medications (Van Putten et al., 1991; Conley and Kelly, 2001). A low plasma level (e.g., less than 5 µg/mL of haloperidol) may require raising the dose or addressing compliance issues. A higher plasma level (e.g., greater than 15 µg/mL of haloperidol) may require lowering the dose because medication side effects may be overshadowing therapeutic effects (Marder, 1997). Other instances when plasma level monitoring may be useful is when decreasing the dose of drug during maintenance therapy, since too low plasma levels may indicate an increased risk of relapse (Marder, 1997).