Critical Reflections on Therapeutic Drug Monitoring

Abstract

THERAPEUTIC DRUG monitoring is fre-.i-quently identified as applied pharmacokinetics. However, this narrow conception is misleading and confines clinical practice to a technical/mechanistic level of intervention rather than to a broader patient-specific focus. Recognizing the need to expand our clinical focus has fueled the clinician's enthusiasm for the application of pharmacodynamics to patient care. However, therapeutic decisions based entirely, or in large part, on serum concentration data are questionable. Posey' observes, &dquo;it appears that at least initially, on a research level, many clinical pharmacists will be turning increasingly to pharmacodynamics instead of serum concentration data to analyze drug regimens.&dquo;Benetl asserts that a correlation between pharmacokinetics and pharmacodynamics will provide more data to our understanding of drugdrug and drug-disease interactions in human populations. However, Benet acknowledges that a correlational data base may not be useful for the treatment of individual patients. For the clinical pharmacist who treats individuals rather than aggregate populations, this is an important consideration.This article describes two major issues that, in spite of the influence of pharmacokinetics and pharmacodynamics, may threaten the responsible practice of patient-specific therapeutic drug monitoring. The two issues are (1) the limitations of the use of aggregate data that are generated from experimental or statistical models and extrapolated by clinicians in order to make individual patient-focused decisions concerning drug therapy, and (2) the dangers inherent in the fragmentation of the patient into technical data values, organ systems, and receptors for drug compounds.- THE PATIENT AS A STATISTICThe most important observation to be made regarding pharmacokinetics, or broader epidemiological data, is that the data are abstract and aggregate and are not individual facts.Thomasma2 observes that, &dquo;[T]he problem of applying general or abstract knowledge to individuals is a central problem in making clinical judgments.&dquo;Thomasma2 describes research and practice as &dquo;logically distinct activities.&dquo; Research attempts have had varying degrees of success in producing generalizable knowledge, but in practice &dquo;therapeutics&dquo; refers to &dquo;a class of activities meant to benefit individual patients or a distinct group of patients.&dquo; In either case, the clinician's ethical mandate is to treat the patient as an individual and not as a member of a group or cohort.Clinicians should keep this research/practice distinction in mind when making clinical decisions, particularly when using pharmacokinetic data that were derived from research results, since these data are often &dquo;applied to individuals who were not part of the original set.&dquo;It is wise to make careful inferences in therapeutics. The more abstract the concept, the less appropriate its application to individual patients. 2The conformity between aggregate data and the individual patien...