[Add-on value of plasma assays during prescription of antipsychotics]

The plasma dosage of antipsychotics is justified in many clinical situations, such as the existence of surprising side effects depending on the dose of administered antipsychotic or, in the event of a lack of therapeutic efficacy. This method allows the clinician to get out of the pure empiricism, trial and error, of his/her supposed beliefs concerning patient compliance and, above all, to base his/her decisions on objective elements in the psychopharmacological field. However, it is clear that this inexpensive and easily accessible technology is not used very often. In the event of a result of the plasma dosage higher or lower than the normal values, it will first be necessary to exclude a phenomenon of drug interactions. Then, the calculation of the metabolic ratio will make it possible to suspect the existence of a particular genetic polymorphism of metabolism (poor metabolizer? ultra-rapid metabolizer?) of the concerned cytochrome, which can then be confirmed by genotyping oriented towards this cytochrome.