What unit is used to express the therapeutic serum concentration range for phenytoin?

• A. mcg/mL
• B. mg/mL
• C. ng/mL
• D. mmol/L

Answer: (A)

Phenytoin’s therapeutic drug level is expressed in micrograms per milliliter because the concentrations that matter clinically are in the tens of micrograms per milliliter. Typically, therapeutic serum levels are about 10–20 mcg/mL, which fits the narrow therapeutic index of phenytoin and the precision needed to avoid toxicity or subtherapeutic dosing. Using nanograms per milliliter would place the value far too low, while milligrams per milliliter would imply unrealistically large amounts for a therapeutic drug level. Millimoles per liter is a molar concentration and isn’t how phenytoin levels are conventionally reported in clinical practice. So mcg/mL is the correct unit.