Description

The measurement of plasma concentrations of drugs given
in therapy (therapeutic drug monitoring, TDM) is useful
for a small number of compounds for which pharmacological
effects cannot be easily assessed and for which the margin
between adequate dosage and potentially toxic dosage is
small. Thus, for some anticonvulsants, notably phenytoin,
anti-infective agents (antimalarial, antimicrobial, and
antiretroviral drugs), cardioactive drugs including digoxin,
most immunosuppressants, and certain psychoactive drugs
(notably clozapine and lithium), TDM may be used to
adjust the dose to individual need and to minimize the risk
of dose-related toxicity. Even for drugs with a wide margin
of safety, TDM may be helpful in assessing adherence to
therapy as a reason for treatment failure.
An appreciation of drug metabolism and of pharmacokinetics,
the study of the rates of drug absorption, distribution, metabolism
and elimination, is essential in understanding the influence of
age, sex, other genetic variables, disease, and other parameters
on the time course and clinical effect of drugs in the body. The
availability of a range of non-isotopic immunoassays compatible
with high-throughput clinical chemistry analyzers has meant
that certain TDM assays are widely available, but in many
cases chromatographic methods (nowadays usually HPLC or
LC-MS) have to be used. Whatever technique is used when
providing a TDM service, adherence to the principles of quality
management (proper method implementation and validation,
and adherence to internal quality control and external quality
assessment procedures) is essential since treatment decisions
may be based on the results.