Therapeutic drug monitoring of individual drugs

Usually, concentration of a therapeutic drug is measured in the serum or plasma. However, whole blood concentration of immunosuppressant drugs such as cyclosporine and tacrolimus is usually measured for therapeutic drug monitoring. Obtaining blood for measurement of a drug during the absorption or the distribution phase may lead to misleading information. Moreover, to measure the peak concentration of a drug, timing of the sample will depend on the route of administration. After intravenous administration, the peak concentration of a drug may be achieved in a few minutes. On the contrary, for a sustained release tablet, the mean time to reach the peak plasma concentration of theophylline was 7.9 h in one study (106). The trough concentration is clinically defined as the serum drug concentration just before the next dose. Usually, trough concentrations are monitored for most drugs, but for aminoglycosides and vancomycin, both peak and trough concentrations are monitored. For a meaningful interpretation of a serum drug concentration, time of specimen collection should be noted along with the time and date of the last dose and route of administration of the drug. This is particularly important for aminoglycoside because without knowing the time of specimen collection, the serum drug concentration cannot be interpreted. Information needed for proper interpretation of drug level for the purpose of therapeutic drug monitoring is listed in Table 4.

Phenytoin, phenobarbital, ethosuximide, valproic acid, and carbamazepine are considered as conventional anticonvulsant drugs. Many people with epilepsy suffer from side effects of anticonvulsants as well as suboptimum seizure control, which can be minimized by regular medication review and dosage adjustments based on serum drug levels (107). All these antiepileptic drugs have a narrow therapeutic range. Phenytoin, carbamazepine, and valproic acid are strongly bound to serum proteins.

Essential Information Required for Interpretation of Serum Concentrations of a Drug

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