Phenytoin, Free, Serum or Plasma

Casandra

Casandra Test Code LC98709Version 1 (DRAFT)

Performing Lab

Lab LabcorpLab Test ID 070763CPT Code 80186

Phenytoin, Free, Serum or Plasma

Also known as: Dilantin-125®, Dilantin®, Free Phenytoin, Phenytek®

Clinical Use

Order Test

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.

Special Instructions

The test measures the free fraction of phenytoin in serum or plasma, distinct from total phenytoin levels which can be affected by binding alterations. Clinicians should draw the specimen just prior to the next phenytoin dose to ensure accurate measurement and reference the therapeutic range of 1.0−2.0 μg/mL. A red-top or green-top (heparin) tube is required, with recommendations against gel-barrier tubes due to phenytoin absorption issues.

Limitations

The use of gel-barrier tubes should be avoided due to potential slow absorption of phenytoin by the gel, which might artefactually decrease drug levels. In addition, total phenytoin levels may not accurately reflect the free drug concentration in conditions like hepatic dysfunction, where phenytoin competes with bilirubin for binding sites. Other drugs, such as valproic acid or ceftriaxone, may displace phenytoin from its binding sites, influencing free levels.

Test Details