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Drug level testing (TDM)
The aim of drug treatment is for the active substance to be present in the body at just the right concentration:
high enough to work, but not so high that it causes side effects or poisoning.
The drug level test – also called therapeutic drug monitoring (TDM) – is a laboratory test that measures the current amount of a medicine in your blood and helps set the dose precisely.
This method is especially important for medicines with a narrow therapeutic range, where the difference between the effect and side effects is minimal.
The test lets the doctor personalise your treatment, preventing unwanted side effects and ensuring the greatest therapeutic effect.
When is drug level testing recommended?
Drug level testing may be needed in the following cases:
- when starting a new medicine, until the optimal dose is found,
- side effects or an insufficient effect occur,
- older patients, or patients with liver or kidney disease, whose breakdown and excretion of medicines may be altered,
- drug interactions are suspected,
- as a regular check during long-term medication (e.g. in the treatment of epilepsy, psychiatric, cardiac or autoimmune conditions).
A precisely set drug level prevents both overdose and lack of effect, so treatment becomes safer and more effective.
What does the lab measure?
From the blood sample, the laboratory measures the plasma concentration of the medicine, that is, how much of the active substance is circulating in your body at the time of the test.
Based on the values, the doctor checks whether the level is within the recommended therapeutic range.
The most commonly monitored groups of medicines:
- antibiotics (e.g. gentamicin, vancomycin),
- antiepileptics (e.g. carbamazepine, valproate, phenytoin, lamotrigine),
- psychiatric medicines (e.g. lithium, antidepressants, antipsychotics),
- heart medicines (e.g. digoxin),
- immunosuppressants – after organ transplantation (e.g. ciclosporin, tacrolimus, sirolimus).
So the test can be not just a check but also life-saving information when it comes to getting the dose of a medicine right.
Sample collection and preparation
For a drug level test, a venous blood sample is taken.
The timing of the sample has to be set according to the type of medicine and how it is dosed, because blood levels can change during the day.
General rules:
- Blood is usually taken just before the next dose (the so-called trough level), when the blood level is at its lowest.
- For some medicines, the doctor may ask for the sample at a set time after the dose, so the peak concentration can also be measured.
- Before the test, it is important that you do not skip your medicine and that you note the exact time you took your last dose.
Taking the sample is quick and painless, and the result is usually ready within a few days.
Evaluating the results
The doctor compares the measured drug level with the therapeutic reference range for that medicine.
Differences may mean:
- a level that is too low – the medicine is not working well enough and the dose should be increased,
- a level that is too high – there is a risk of side effects or poisoning,
- fluctuating values – which may point to an absorption or interaction problem.
Based on the laboratory results, your doctor may change the dose of the medicine or how often you take it, ensuring the right therapeutic effect and your safety.
The role of drug level testing in personalised medicine
Everyone’s body reacts to medicines differently.
Age, body weight, metabolism, liver and kidney function, and even genetics affect how a medicine is broken down and how quickly it leaves the body.
Drug level monitoring is one of the most important tools of precision medicine:
it lets the doctor personalise your treatment, keeping side effects to a minimum while the treatment stays as effective as possible.
At Sabjanics Medical Center we carry out drug level tests using high-precision laboratory methods, so that our patients’ treatment is always safe, balanced and effective.
