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 will be monitored 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 this range, most people will be effectively treated without excessive side effects or symptoms of toxicity. The drug dosage to reach this level must be individually determined. When a patient starts on a monitored drug (or returns to it after an absence), the doctor adjusts the dose upwards and tests blood concentrations frequently until the appropriate steady state level is achieved. If a patient's levels are too high, the doctor will adjust them lower. 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 patients work closely with their doctors 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 patient's results are in the therapeutic range and their clinical signs indicate that the treatment is appropriate, then the doctor 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 patient has either excessive side effects or signs of toxicity, then testing will be done adjust the drug dosage and maintain levels within the therapeutic range. Sometimes, the doctor may need to re-evaluate the use of a specific medication and consider switching to another type of drug to better fit the patient'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 (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, doctors know when to expect peaks and troughs to occur and will request blood sample collections as either trough levels (usually drawn just before the next dose), peak levels (timing varies depending on the drug), or sometimes a randomly timed level. Consistent and accurate interpretation of the results depends on the timing of sample collection. If a patient is unable to take their medication and have their blood drawn at the appropriate time interval, then they should talk to their doctor before the sample is collected.