Medicine Digests
Therapeutic drug monitoring is based upon the collaboration between a health care provider (clinician, pharmacist, nurse) responsible for making quantitative and qualitative decisions about drug treatment and the clinical laboratory providing analytical services for the measurement of drug concentrations. The information provided by a drug concentration measurement is generally greater than for other substances measured by the laboratory.
This is because, unlike say sodium or glucose, the intake of a drug is quite well known and the processes of distribution and elimination are usually very simple and not under the control of a multitude of homeostatic controlling reflexes.
Given an accurate dosing history and one or more drug concentrations, it is possible to describe the pharmacokinetic processes of distribution and elimination quite precisely in an individual patient and to make accurate predictions of concentrations at future points in time, whatever dosing regimen is used.
The ability to interpret drug concentrations and extract information so that future concentrations can be predicted and a rational dosing scheme instituted, requires a different kind of intellectual effort from that needed to interpret, say, a serum glucose concentration. The latter is mostly interpreted by reference to a so-called ‘normal range’ – usually the 95% confidence interval based upon measurements from a sample of a ‘normal’ population.
Under most circumstances, if the serum glucose concentration is within the “normal range”, little further attention is paid to it If it is outside of the ‘normal range’, diagnostic efforts are made to determine what pathophysiological process is disturbed. But the precise value will be used only in a semi quantitative fashion, eg. high, very high, or extremely high, or in reference to some previously defined diagnostic threshold value; for example, diabetes mellitus may be diagnosed if the glucose concentration is greater than 10 mmol/L.
On the other hand, all drug concentrations exceed the “normal” range because it would be abnormal to be able to detect any therapeutic substance in the serum of a normal, healthy individual.
The quantitative information provided by a drug concentration measurement can be usefully applied with up to 2 significant figures in the determination of, say, drug clearance. This degree of precision is implicit in use of this information because individual dosage decisions will often be made with 2 significant figures in the dose size.
The quantitative approach to therapeutic decision making is relatively new to the art of medicine. Most clinicians currently practicing medicine will not have been taught very much pharmacokinetics during their undergraduate training and may therefore have only a very hazy idea of the quantitative decisions that form the basis of rational therapeutics.
Furthermore, younger clinicians may be misled into thinking that the pharmacokinetics they were taught at medical school are irrelevant to modern medicine because their senior colleagues pay no attention to pharmacokinetic detail and make therapeutic decisions in a seemingly capricious fashion. Such a conclusion may be quite false because therapeutic decisions made by an experienced clinician are founded upon a wide base of knowledge gained from treating many similar patients.
This prior knowledge of the characteristics of the population being treated provides an empirical, but nevertheless frequently satisfactory, guide to making an appropriate quantitative and qualitative therapeutic decision. Recognition of the value of such prior information when faced with an individual patient about whom little is known is the basis of the latest techniques of rational, quantitative pharmacokinetic and pharmacodynamic forecasting.
For the person who must make a therapeutic choice on behalf of an individual patient, the application of quantitative pharmacokinetic principles can substitute for the advice of a more experienced colleague. For all clinicians, young and old, these same principles can be applied to new therapeutic entities and reduce the suffering of patients who would otherwise be exposed to the vagaries of a trial and error approach.
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