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What is therapeutic drug monitoring?
Therapeutic drug monitoring is ns measuring the level of some drugs 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 use another test (like blood pressure, temperature, or glucose) 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 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 medication, there may be interactions between the drugs that affect the way the body absorbs or metabolizes 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:
Drug Category |
Drugs in that Category |
Treatment Use
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Cardiac drugs |
Digoxin, quinidine, procainamide, N-acetyl-procainamide (a metabolite of procainamide) |
Congestive heart failure, angina, arrhythmias
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Antibiotics |
Aminoglycosides (gentamicin, tobramycin, amikacin)
Vancomycin,
Chloramphenicol |
Infections with bacteria that are resistant to less toxic antibiotics |
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Antiepileptics |
Phenobarbital, phenytoin, valproic acid, carbamazepine, ethosuximide,
sometimes gabapentin, lamotrigine |
Epilepsy, prevention of seizures |
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Bronchodilators |
Theophylline, caffeine |
Asthma, Chronic obstructive
pulmonary disorder (COPD), neonatal apnea |
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Immunosuppressants |
Cyclosporine, tacrolimus, sometimes sirolimus, mycophenolate mofetile |
Prevent rejection of transplanted organs |
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Anti-cancer drugs |
Methotrexate |
Various cancers, rheumatoid arthritis, non-hodgkin's lymphomas, osteosarcoma, psoriasis |
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Psychiatric drugs |
Lithium, desipramine, some antidepressants (imipramine, amitriptyline, nortriptyline, doxepin) |
bipolar disorder (manic depression), depression |
How does TDM work?
Through years of testing, the optimum blood level range for each drug has been determined. (Remember, though, that each of us is different and the best drug level for one person may be different from another person.) For most of the drugs listed, blood is drawn right before a dose is given. For some drugs, the blood collection will take place after the dose is given at a specific time (for example, 90 minutes later); this time will vary and may be as long as 8 hours after dosing). Drawing 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 scheduled to have your blood drawn for a drug level, make sure you know if you are to take the drug before or after the blood draw is done. If you forget and do the opposite (take the drug when you are supposed to wait), let someone in the doctor’s office or lab know before the blood is collected.
FAQs
1. How does the doctor determine how much drug to give me?
There are many factors to consider. Some of them are your weight, body composition, age, temperature, gastric movement, nutrition, renal, liver, and heart conditions, burns, shock, and trauma.
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 done on special laboratory equipment.
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