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THERAPEUTIC DRUG MONITORING

Measuring Drug Levels in Blood

The ability to accurately measure how much medicine is in your blood has evolved since the early 1970s. People vary greatly in nature and degree of their responses to drugs, and blood levels can help find the best dose. For many drugs, the clinical response clearly shows that the drug is working as intended. However, for some drugs-especially those with narrow safety margins-toxic reactions may closely resemble the symptoms being treated. In many cases, the patient's expected response is not in keeping with his or her clinical condition. By measuring blood levels at appropriate times, the physician or clinical pharmacist can adjust dosing schedules more accurately, reduce the risk of toxicity, and achieve the best results or outcomes.

Some medications require both a peak and trough level to ensure best results, and timing of blood sampling is critical. Sampling should be avoided during the two hours after an oral dose because during this absorption period, blood levels do not represent tissue levels of the drug, and the tissue is where many medicines actually work. The peak, or highest, level of the drug can measure several things: how effectively bacteria are killed or toxic levels within the body, for instance. The trough, or lowest, level tells how effectively a medicine is cleared from the body between doses. This can also be important because if too much drug remains toxicity can result, and if too little, it may not work well.

The following drugs are those most suitable for therapeutic drug monitoring. If you are using any of these on a regular basis, ask your doctor about checking blood levels. These numbers are ranges where effects are usually seen, but people react differently. Therapeutic ranges listed are not absolute fixed levels where lack of effect or toxicity definitely occur. Some have a therapeutic response at a level lower than the low end of the range. Generally in this case, the level should be followed closely and left alone. A blood level higher than the high end of the range is not always toxic, but is a level higher than desirable, and dosing changes should be made. Fortunately most physicians "treat the patient, not the level".