Therapeutic Drug Monitoring

Everybody deserves to be treated differently 

What is TDM?

Therapeutic drug monitoring (TDM) is the clinical practice of measuring specific drugs at designated intervals to maintain a constant concentration in a patient’s bloodstream, thereby optimizing individual dosage regimens.

In other words, TDM refers to the individualization of drug dosage by maintaining plasma or blood drug concentrations within a targeted therapeutic range or window.

Therapeutic drug monitoring involves not only measuring drug concentrations, but also the clinical interpretation of the result.

The goal of this process is to individualize therapeutic regimens for optimal patient benefit. By combining knowledge of pharmaceutics, pharmacokinetics and pharmacodynamics, TDM enables the assessment of the efficacy and safety of a particular medication in a variety of clinical settings. TDM is based on the principle that for some drugs there is a close relationship between the plasma level of the drug and its clinical effect.

Another critical assumption is that drug metabolism varies from patient to patient. When a precise therapeutic end point is difficult to define, monitoring of drug levels may be of considerable therapeutic assistance. Routine monitoring is however not advocated for most drugs. Only clinically meaningful tests should be performed.

Therapeutic drug monitoring aims to promote optimum drug treatment by maintaining serum drug levels within a therapeutic range. 

When is TDM needed?

The science of Therapeutic Drug Monitoring grew out of the recognition that:

  • Certain drugs have a narrow therapeutic range.
  • In concentrations above the upper limit of the range, the drug can be toxic.
  • In concentrations below the lower limit of the range, the drug can be ineffective.
  • Not all patients have the same response at similar doses.

Studies have determined the clinical value of TDM testing, and in certain instances clear clinical value was demonstrated.

Today there are over 20 therapeutic drugs which are routinely monitored.

Criteria for TDM

1) An appropriate analytical test for drug and active metabolites must exist.

2) The drug should have a narrow therapeutic range.

3) Patients are not showing adequate clinical response to a drug despite being on adequate dose.

4) The therapeutic effect can not be readily assessed by the clinical observation (e.g. anticonvulsants, antidepressants etc.)

5) There is a large individual variability in steady state plasma concentration exists at any given dose.

When should TDM be applied?

While there may be specific individual circumstances, most clinical indications for TDM can be summarized as follows:

1. The therapeutic index is low.

2. The clinical endpoint is poorly defined.

3. There is non compliance.

4. In case of therapeutic failure.

5. In drugs with saturable metabolism.

6. When there is a wide variation in the metabolism of drugs.

7. In diagnosis of suspected toxicity & determining drug abuse.

8. In drugs with steep dose response curve can result in a marked increase in desired/undesired responses.

9. When another drug alter the relationship between dose & plasma concentration e.g. plasma concentration of lithium is increased by thiazide.

10. In case of renal disease (it alters the relationship between dose & the plasma concentration). 


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