28 Monitoring Drug

Concentrations in Clinical

Practice

Anthony W. Fox

28.1 General principles of

therapeutic monitoring

Measuring drug concentrations, for example, in
urine or blood is quite a small part of the general,
laudable clinical goal of monitoring therapy.
Therapeutic monitoring comes in many guises,
ranging from the absence of reattendance of the
patient with the same complaint (presumptive but
not certain treatment success!), through various
types of quantitative efficacy monitoring (e.g.
activated prothrombin time and/or partial throm-
boplastin time for warfarin therapy; Nowak,
2001). Clearly, all prescribing should be accom-
panied by some sort of therapeutic monitoring,
and good product labeling will explain how.
While the pursuit of pharmacokinetic informa-
tion is an obligatory part of drug development in
general, this chapter focuses on the situation
when drug concentration measurements become
part of ordinary clinical practice. Above all, even
when drug concentration has been measured,
treating the patient rather than the laboratory
report is paramount; no drug has a single target
plasma concentration.

28.2 Why monitor drug

concentrations?

The following reasons can justify the need to moni-

tor drug concentrations in plasma or urine:

Avoidance of adverse effects for drugs with

narrow ‘therapeutic windows’

Maximizing probability of efficacy (e.g. avoid-

ing too low a dose of a prophylactic drug)

Checking for compliance

Detection of exposure (e.g. environmental risk

studies; Lange and Dietrich, 2002)

Treatment decision making (e.g. paracetamol/

acetaminophen overdose)

Avoidance of drug interactions

Dose adjustment for special populations (e.g. the

elderly, children, renal failure)

Principles and Practice of Pharmaceutical Medicine, 2nd Edition Edited by L. D. Edwards, A. J. Fletcher, A. W. Fox and P. D. Stonier
#2007 John Wiley & Sons, Ltd ISBN: 978-0-470-09313-9