Use

Phenytoin is primarily protein-bound, with the free fraction being biologically active and crossing plasma membranes. Testing for the free phenytoin level is particularly relevant in specific clinical settings where altered phenytoin binding occurs, such as in patients with renal or hepatic disease, or in those experiencing polypharmacy, pregnancy, or a hypermetabolic state. Understanding the free fraction can assist in drug dosing, particularly if total phenytoin levels are misleading due to altered binding dynamics. Evaluation of free phenytoin levels can be crucial in cases of suspected toxicity or overdosage, guiding treatment decisions, including interventions like dialysis.