NAPLEX Question of the Week: Therapeutic Drug Monitoring

Pharmacists are involved with therapeutic drug monitoring often on a daily basis within the inpatient setting. Today's question examines normal therapeutic concentrations of several medications.

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Place the following medications in order of lowest to highest normal therapeutic drug concentration:

A. Phenytoin (Total concentration)

B. Digoxin

C. Cyclosporine (Trough concentration)

D. Valproic Acid (Total concentration)

Answer with rationale: The correct order is B-C-A-D. This question is a bit tricky if you don't know the units of concentration that is normal for each agent. Digoxin and cyclosporine are both measured in nanograms/mL while phenytoin and valproic acid are measured in mcg/mL. Therefore digoxin and cyclosporine are definitely lower than phenytoin and valproic acid. Digoxin's normal concentration is around 0.8-2 ng/mL targeting more on the lower end for heart failure patients. Cyclosporine depending on indication for transplant rejection prevention will be somewhere around 150-200 ng/mL. Phenytoin (total concentration) will be around 10-20 mcg/mL while valproic acid will be around 50-100 or 125 mcg/mL. It is important to remember that many questions will be case based so recognizing abnormal lab values is crucial in order to correctly answer questions on the exam.

Keep studying hard and congrats to those who have passed!

Dr. B

Christopher M. Bland

Clinical Associate Professor, University of Georgia College of Pharmacy

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