What was the measured phenytoin level after intravenous administration?

• A. 10.4 mcg/mL
• B. 5.2 mcg/mL
• C. 15.0 mcg/mL
• D. 20.4 mcg/mL

Answer: (A)

Phenytoin requires careful therapeutic drug monitoring because it has a narrow range where it’s effective without being toxic. For total phenytoin, the common therapeutic window is about 10 to 20 micrograms per milliliter. A measured level of 10.4 mcg/mL sits inside that range, which means the IV dose achieved a therapeutic concentration suitable for controlling seizures without excessive exposure. Levels well below 10 can leave seizures uncontrolled, while levels around or above 20 raise concern for toxicity. Remember that factors like low albumin can affect interpretation, since less phenytoin remains bound and the free (active) portion can be higher than the total level suggests.