Limitation of Therapeutic Drug Monitoring

·         There may be some factors that may lead to incorrect interpretation of plasma drug levels such as;

o   Non-compliance of patient leading to low plasma drug levels

o   Low dose administered which leads to subtharapapeutic concentrations

o   Patient suffer from mal-absorption

o   Drug with low bioavailability may also lead to subtherapeutic concentrations

o   Concomitant drugs that could affect the plasma levels of the drug in question

o   Hepatic or renal dysfunction

o   Diseases related to genetic factors affecting drug metabolism

o   .Time of administration of the drug is not accurate.

o   Dose administration error.

o   Inaccurate time of sampling, or timing was before steady-state is reached 

o   Wrong site of sampling.

o   Lab assay error.

·         Effect of age

o   There is a great variability in response to drugs at extremes of age.

o   As an example, elderly patients are more sensitive to the CNS depressant effect of drugs but are less sensitive to cardiovascular effects of Propranolol.

o   It is well known that children are more sensitive to morphine.

·         Pregnancy

  • Many drugs can be affected by pregnancy state.

  • As an example, drug levels of phenytoin and phenobarbitone are lower during pregnancy


Wells BG, DiPiro J, Schwinghammer T (2013), Pharmacotherapy Handbook (6th Ed). New York, NY: McGraw-Hill.

DiPiro JT, Talbert RL, Yee GC, Matzke GR, Wells BG, Posey ML, (2008): Pharmacotherapy: A Pathophysiologic Approach (7th ed): New York, NY: McGraw-Hill.

Katz M D., Matthias KR., Chisholm-Burns M A., Pharmacotherapy(2011) Principles & Practice Study Guide: A Case-Based Care Plan Approach: New York, NY: McGraw-Hill.

Schwinghammer TL, Koehler JM (2009) Pharmacotherapy Casebook: A Patient-Focused Approach (7th ed): New York, NY: McGraw-Hill.

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