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Therapeutic Drug Monitoring

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How are these tests used?

Not all drug levels need to be monitored. These tests are used to monitor blood levels of particular drugs that have a narrow range in which the drug is effective but not toxic. In addition, some drugs require monitoring because the amount of drug given does not correlate well with the amount of drug that may reach the bloodstream. Sometimes, the way that a particular drug is absorbed and metabolized can vary from person to person, or the physical or health status of a person can affect the drug level in the blood.

Through years of testing, the optimum therapeutic ranges for drugs have been determined. In these ranges, most people will be effectively treated without excessive side effects or symptoms of toxicity. The drug dosage necessary to reach this level must be determined for each individual. When a person starts on a monitored drug (or returns to it after an absence), the health practitioner adjusts the dose upwards and tests blood concentrations frequently until the appropriate steady level is achieved. If someone's levels are too high, the health practitioner will lower the dosage. Often, each different dosage level will take a short period of time to stabilize, so these corrections up and down may take place over a few days or weeks. It is important that people work closely with their health practitioner during this process and not make their own adjustments or stop taking their medication. Abrupt changes can sometimes worsen conditions and cause acute symptoms.

When are they ordered?

Levels of monitored drugs are often tested frequently when a person is first put on a drug regimen. Once a person's results are in the therapeutic range and his or her clinical signs indicate that the treatment is appropriate, then the health practitioner may monitor the drug at regular intervals and as needed to accommodate changes in patient status and to ensure that the drug stays in the therapeutic range. The frequency of testing required will depend on the drug and on the needs of the patient. If treatment does not appear to be fully effective, or if the person has either excessive side effects or signs of toxicity, then the health practitioner will order testing aimed at adjusting the drug dosage and maintaining levels within the therapeutic range. Sometimes, the health practitioner may need to re-evaluate the use of a specific medication and consider switching to another type of drug to better fit the person's condition.

The timing of blood collection is an important part of therapeutic drug monitoring. When a person takes a dose of drug, the amount in the blood rises for a period of time, peaks, and then begins to fall, usually reaching its lowest level, or trough, just before the next dose. To be effective, peak levels should be below toxic concentrations and trough levels should remain in the therapeutic range. Through experience and studies, health practitioners know when to expect peaks and troughs and will request blood sample collections as either trough levels (usually drawn just before the next dose), peak levels (for which timing varies depending on the drug), or sometimes as a randomly timed level. Consistent and accurate interpretation of the results depends on the timing of sample collection. If someone is unable to take his or her medication or have blood drawn at the appropriate time interval, then he or she should talk to their health practitioner before the sample is collected.

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