Pharmacological response of a drug given in a selected dosing regimen depends on several factors, including compliance of the patient, bioavailability of the drug, rate of drug metabolism (depending on the genetic make up of the patient) as well as the protein-binding ability of the drug. It is well established that only unbound (free) free drug can bind with the receptor and produce the desired effect. For certain drugs, a good correlation exists between serum drug concentrations and pharmacological response. Therefore, monitoring serum concentrations of these drugs is beneficial for patient management especially if the drugs have very narrow therapeutic ranges. Moreover, for these drugs, adjustment of dosing may be more useful based on serum drug concentration rather than routine assessment of a patient. For example, adjusting phenytoin dosing in patients based on their serum phenytoin concentrations rather than seizure frequencies not only decrease the morbidity but also prevent unnecessary toxicity of phenytoin in these patients. Peterson et al. (1) reported that in their study with 114 patients, total phenytoin concentrations provided as good an indication of the clinical response as the free phenytoin concentrations in most patients, but in 14.2% patients, free phenytoin concentrations were better correlated with clinical picture than total phenytoin concentrations. Another report indicated that quality of life improved in a group of patients with congestive heart failure where digoxin dosing was based on target therapeutic concentrations (2).

Therapeutic drug monitoring has been used in clinical practice to individualize drug therapy since the beginning of the 1970s. The goal of therapeutic drug monitoring is to optimize pharmacological responses of a drug while avoiding adverse effects. Usually for drugs that are routinely monitored in clinical laboratories, serum concentrations are a better predictor of desired pharmacological effects than the dose. Moreover, therapeutic drug monitoring is also utilized to monitor a patient's compliance with a drug regimen and to identify potential drug-drug or food-drug interactions.

Therapeutic drug monitoring not only consists of measuring the concentration of a drug in a biological matrix but it also involves the proper interpretation of the value using pharmacokinetic parameters, drawing appropriate conclusion regarding the drug concentration and dose adjustment. The International Association for Therapeutic Drug Monitoring and Clinical Toxicology adopted the following definition, "Therapeutic drug monitoring is defined as the measurement made in the laboratory of a parameter that, with appropriate interpretation, will directly influence prescribing procedures. Commonly, the measurement is in a biological matrix of a prescribed xenobiotic, but it may also be of an endogenous compound prescribed as a replacement therapy in an individual who is physiologically or pathologically deficient in that compound" (3).

Only a fraction of drugs currently used worldwide require routine monitoring. For a drug in which the gap between therapeutic and toxic concentration is wide, therapeutic drug monitoring is not indicated unless in the case of intentional overdose, for example with salicylate or acetaminophen. In an Italian collaborative study on the utilization of therapeutic drug monitoring, it was noted that only 16.3% of the population was given drugs for which therapeutic drug monitoring was available in the hospital. Digoxin was the most frequently ordered drug in their study population (4).

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