What is therapeutic drug monitoring?

Therapeutic drug monitoring (TDM) is the term used for measuring the level of some drugs at timed intervals as a way to determine the most effective dose or to avoid toxicity. Most drugs do not need to be monitored this way because your doctor can look for an improvement in symptoms or use tests like blood pressure, temperature, or blood glucose to determine if the dose is correct.

Why is it important?

The drugs that are required to be monitored have some special features: most of them work best over a small range – below this range, the drug is not effective and the patient begins having symptoms again; above this range, the drug has bad or toxic side effects that you want to avoid. Maintaining a steady concentration in the blood is not as simple as giving a standard dose of medication to everyone. Each person will absorb, metabolise, utilise, and eliminate drugs at different rates based upon their age, general state of health and genetic makeup. These factors may change over time and vary from day to day or with various disease states.

Since many people take more than one medication, there may be interactions between the drugs that affect the way the body absorbs or metabolises one of them. Also, some patients do not take medications as prescribed or instructed. Monitoring identifies these cases.
 

What drugs are monitored?
There are several categories of drugs that require monitoring, as summarised here:
 
Drug category Drugs in that category Treatment use
Cardiac drugs Digoxin Congestive heart failure, abnormal heart rhythms (arrhythmias)
Antibiotics Aminoglycosides (eg.gentamicin) Infections with bacteria that are resistant to less toxic antibiotics
Antiepileptics Phenytoin, valproate, carbamazepine Epilepsy, prevention of seizures
Immunosuppressants Cyclosporin, tacrolimus Prevent rejection of transplanted organs
Anti-cancer drugs Methotrexate Various cancers, rheumatoid arthritis, psoriasis
Psychiatric drugs Lithium Bipolar disorder (manic depression), depression
How does TDM work?

For most of the drugs listed, blood is collected immediately before a dose is given. For some drugs, the blood collection will take place after the dose is given at a specific time after taking the drug.

Collecting the blood at the correct time is very important. The amount of drug measured in the blood will be used to determine if a patient is getting the right amount of the medication and, in some cases, to calculate the amount of drug that will be given the next time. So, if you are having a sample taken for measurement of a drug level, make sure you know if you are to take the drug before or after the sample has been collected. If you forget and do the opposite (take the drug when you are supposed to wait), let your doctor know.

Common questions 

1. How does the doctor determine how much drug to give me?
There are many factors to consider. These include your weight and age and the presence of any kidney, liver or heart conditions.

2. What should I do if I forget to take my medication on time?
Do not double your dose the next time. Consult your doctor or pharmacist to find out what you should do.

3. Can I monitor myself at home?
No. Blood must be collected at specific times and tests performed on special laboratory equipment.


Last Review Date: January 11, 2023


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