Interpreting plasma concentration measurements

The following points are relevant:

• A target therapeutic concentration range quoted for a drug should be regarded only as a guide to help to optimise dosing and should be evaluated with other clinical indicators of progress.

• Consider whether a patient has been taking a drug for a sufficient time to reach steady-state conditions, i.e. when 5 \}/2 periods have elapsed since dosing commenced or since the last change in dose. In the case of drugs that alter their own rates of metabolism by enzyme induction, e.g. carbamazepine and phenytoin, it is best to allow 2-4 weeks to elapse between change in dose and plasma concentration measurement. Sampling when plasma concentrations are still rising or falling towards a steady state is likely to be misleading.

• Consider whether peak or trough concentration should be measured. As a general rule when a drug has a short t\ it is desirable to know both; monitoring peak (15 min after an i.v. dose) and trough (just before the next dose) concentrations of gentamicin (t^ 2.5 h) helps to provide efficacy without toxicity. For a drug with a long t^, it is usually best to sample just before a dose is due; effective immunosuppression with ciclosporin (ty2 27 h) is obtained with trough concentrations

Commonsense considerations of anatomy, physiology, pathology, pharmacology, therapeutics and convenience determine the routes by which drugs are administered. Usually these are:

• Enteral: by mouth (swallowed) or by sublingual or buccal absorption; by rectum

• Parenteral: by intravenous injection or infusion, intramuscular injection, subcutaneous injection or infusion, inhalation, topical application for local (skin, eye, lung) or for systemic (transdermal) effect

• Other routes, e.g. intrathecal, intradermal, intranasal, intratracheal, intrapleural, are used when appropriate.

The features of the various routes, their advantages and disadvantages are relevant.

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