· 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.