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

References
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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