Therapeutic Drug Monitoring

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What is therapeutic drug monitoring?
Therapeutic drug monitoring is the term used for measuring the level of some drugs as a way to find out 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 lab tests to tell if the dose is right.

Why is it important?
The drugs that are monitored have some special features: most of them work best over a small blood concentration 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. Since many people take more than one drug, 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 drugs as prescribed or instructed. Monitoring can identify these cases.

What drugs are monitored?

Monitored Drugs by Category

There are several categories of drugs that require monitoring, as summarized here.
Drug CategoryDrugsTreatment Use
Cardiac drugs Digoxin Congestive heart failure, angina, arrhythmias
Antibiotics Aminoglycosides (eg gentamicin) Infections with bacteria that are resistant to less toxic antibiotics
Antiepileptics Phenobarbitone, phenytoin, valproate, carbamazepine Epilepsy, prevention of seizures
Bronchodilators Theophylline, caffeine Asthma, Chronic obstructive pulmonary disorder (COPD), neonatal apnoea
Immunosuppressants Ciclosporin, tacrolimus, and others Prevent rejection of transplanted organs
Anti-cancer drugs Methotrexate Psoriasis, rheumatoid arthritis, various cancers
Psychiatric drugs Lithium Bipolar disorder (manic depression), depression

How does TDM work?
Through years of testing, the best blood level range for each drug has been found although, because each of us is slightly different the best drug level may be different for one person than another. For most of the drugs listed, blood is taken right before a dose is given. For some drugs, the blood collection will take place at a specified time after taking the drug.  This time may be a short period of perhaps, 90 minutes or may be as long as 8 hours. Collecting the blood at the correct time is very important. The amount of drug measured in the blood will be used to find out if a patient is getting the right amount of the drug and, in some cases, to calculate the amount of drug that needs to be given the next time. It is therefore important  if you are having a sample taken for measurement of a drug level, to 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), ask your doctor or the laboratory.

1. How does the doctor work out how much drug to give me?
There are many factors to consider. These include your weight, age, 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. Ask your doctor or pharmacist to find out what you should do.

3. Can I monitor the drugs myself at home?
No. Blood must be collected at particular times and tests done on special laboratory equipment.

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Conditions: Kidney disease, Liver disease, Cardiovascular disease