Therapeutic Drug Monitoring (TDM) in psychiatry (part I): why studies attempting to correlate drug concentration and antidepressant response don't work.

In this column, the first in a series discussing why therapeutic drug monitoring (TDM) is a seriously underutilized tool in psychiatry, the author explains why standard antidepressant registration trials are not able to establish a correlation between antidepressant response and the plasma concentration of biogenic amine antidepressants, such as selective serotonin reuptake inhibitors and serotonin-norepinephrine reuptake inhibitors. The problem is that such studies have a poor signal-to- noise ratio. In such studies, approximately one third of participants receiving drug respond specifically because of the drug, one third of participants receiving drug respond not because of the drug but rather because of the "placebo" effect inherent in participating in such a study, and one third of participants on drug do not respond sufficiently to be counted as responders. In analyzing the results of such studies, the data from these last two groups make it impossible to identify whether there is any relationship between drug concentration and antidepressant response. The next column in this series will discuss how TDM can be used as a "personalized medicine" tool to evaluate patients who are at risk for less than optimum response either because they may have much more rapid or much slower clearance of a drug than is usual as well as to identify adherence problems.