Free phenytoin toxicity.

Phenytoin has a narrow therapeutic window, and when managing cases of toxicity, clinicians are very wary of this fact. Typically, if patient presents with symptoms suggestive of phenytoin toxicity, total serum phenytoin is promptly ordered. That could be falsely low especially in elderly or critically ill patients, which may lead to a low albumin level resulting in this discrepancy. The free phenytoin can be best estimated using the Sheiner-Tozer equation. Herein, we describe a case of an elderly male patient who presented with drowsiness, gait changes, and elevated liver enzymes and a normal total serum phenytoin level of 18 ng/dL (normal, 10-20 ng/dL).After taking his albumin level into account, his free phenytoin level was calculated to be 27 ng/dL, and the phenytoin was discontinued leading to resolution of his symptoms as well as a return of his liver function panel values to baseline.