Therapeutic Drug Monitoring (TDM), Drug Interactions checking programs and pharmacogenetic tests are valuable instruments in daily clinical practice's quality assurance. However, routine testing would be expensive and there seems no need for it. Many years work experience in pharmacovigilance and quality assurance in psychiatry led us to develop and apply an algorithm [1]. This decision tree leads in several ,,stop/go" steps from the clinical situation of inefficacy or adverse reaction to measuring and interpreting plasma levels, checking for pharmacokinetic interactions and finally, if indicated, to pharmacogenetic tests with genotyping and/or phenotyping. Genetic results should be noted for future treatments, a ,,pharmacogenetic card" is proposed.
Several clinical cases, mainly from the pharmacovigilance program AMSP, are used to illustrate its practical use: interpretation of plasma levels and metabolic ratios, benefit and limitations of drug interaction programs, indication for pharmacogenetic testing, and examples of other factors influencing drug response.
The interplay of genetics, drug interactions, life style and other personal vulnerabilities like comorbidity make prediction of drug response very complex. The use of TDMplus has proven useful guiding the clinicians in difficult clinical situations and helping elucidating the causality of adverse drug reactions.