What is the therapeutic range of phenytoin in mcg/mL?

What is the therapeutic range of phenytoin in mcg/mL?

• A. 0.5-1.5
• B. 5-15
• C. 10-20
• D. 30-40

Answer: (C)

Phenytoin works best when its level in the blood is in a specific window where it controls seizures but avoids toxicity. The commonly accepted therapeutic range is 10–20 mcg/mL. Monitoring trough levels helps ensure you stay within this range, because phenytoin metabolism becomes saturated at higher concentrations, so small dose increases can cause larger-than-expected rises in blood level. Levels below 10 mcg/mL may be subtherapeutic and risk ongoing seizures, while levels above roughly 20–30 mcg/mL increase the likelihood of side effects like nystagmus, ataxia, and confusion. The other options are either too low to be effective or clearly toxic.

Phenytoin works best when its level in the blood is in a specific window where it controls seizures but avoids toxicity. The commonly accepted therapeutic range is 10–20 mcg/mL. Monitoring trough levels helps ensure you stay within this range, because phenytoin metabolism becomes saturated at higher concentrations, so small dose increases can cause larger-than-expected rises in blood level. Levels below 10 mcg/mL may be subtherapeutic and risk ongoing seizures, while levels above roughly 20–30 mcg/mL increase the likelihood of side effects like nystagmus, ataxia, and confusion. The other options are either too low to be effective or clearly toxic.