Wednesday, April 24, 2019

Therapeutic drug monitoring intro

To most fresh provisional registered pharmacists who entered therapeutic drug monitoring (in short, TDM), TDM is about

  • calculation using formula
  • withold or continue the drug based on drug level, and finally
  • recommendations of standard monitoring parameter.
Well, it sums up quite well in a way. However, to me, there is a few fundamental concepts that we should touch on.

First and foremost, we need to have an understanding of why we do TDM. I personally see the TDM playing three important roles: 
  • First, it is to ensure the narrow therapeutic window drug level is within therapeutic range. If the drug level is below or above the range, it will serve as a guide for dosage adjustment. However, the result should be interpreted in a clinical context. To illustrate, we would not adjust the dose of antiepileptic drug if the patient is seizure free despite has a drug level lower than therapeutic range.
  • to check patient compliance to a drug, for example when we do a random sampling of an antiepileptic.
  • to check for drug toxicity if suspected, for example paracetamol poisoning.
Every one of us is very different from each other, including how our body react to an administered drug. Scientifically speaking, there could be an interpatient variability in the dose-regimen relationship or what we termed as "pharmacokinetic difference”. When a patient is started on a narrow therapeutic window drug, the dosing will be based on population data. It is only after doing a TDM, we might be able to draw out a more precise estimation based on the individual patient profile. In real practice, the estimated value does not equivalent to the actual measured value. Hence, resampling of drug is unavoidable still.

In TDM, estimation using formula is very important still. I used to think that there is no need for me to guide you on calculation since you are just playing around the formula given. However, past experience taught me otherwise. If a wrong formula is being utilized, wrong estimation will be made and this will be followed by wrong recommendations, such as unnecessary resampling. Hence, an appreciation should be given to the TDM formulae, instead of blindly using them.

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