How does TDM work?
Through years of testing, the optimum therapeutic blood level range for each drug has 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 (although if they are causing symptoms associated with toxicity, they will be decreased relatively rapidly to relieve these symptoms). 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.
Once the 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 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 to see if blood concentrations have become too low or high. If they have, then the dosage will be adjusted; if they have not, then the patient and doctor may need to re-evaluate the use of that specific medication and consider switching to another type of drug if it is available.
The timing of blood collection is an important part of therapeutic drug monitoring. When a person takes a dose of a drug, the amount in the blood rises for a time period, 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 will request a random 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.
