Drug concentrations in neuropsychiatry: alternative approaches.

The practical value of measuring plasma concentrations is to optimize treatment in order to attain the best balance in each particular patient between wanted clinical therapeutic effects and unwanted adverse and side-effects. This increase in treatment efficacy can be concurrent--that is, the treatment is monitored and the dosage changed as necessary--or predictive, where the results of a test dose can be used to calculate the appropriate dosage. Wanted effects include: (1) the attainment of adequate drug concentrations for the minimum period of time necessary, as in the use of antibiotics; (2) the suppression of symptoms, as in the use of the benzodiazepines in anxiety states; (3) the use of drugs to improve function, e.g. levodopa in Parkinsonism; (4) drugs are used to prevent episodes of illness; and (5) complex suppression of symptoms, as with the antidepressives and antipsychotics. The biological alternatives to plasma level monitoring depend on establishing an empirical relationship between the putative monitor and the clinical response. Such measures avoid the problem of estimation of drug concentration at the receptor. Examples include monoamine oxidase activity in platelets, the uptake of amines into platelets, neuroendocrine measures such as prolactin concentrations, the electroencephalogram, and peripheral measures such as pulse rate, pupil size and tremor.