PHENYTOIN, FREE

Message

Collect as a trough just prior to next dose. For patients receiving fosphenytoin therapy, collect as a trough at least 2 hours after IV infusion or at least 4 hours after IM injection.

Test Code

LAB32

CPT Codes
80186

Preferred Specimen

One 6 mL red top tube
Pediatric: One red top microtainer

Other Acceptable Specimens

Dark green top tube

Instructions

Specify dosage history. Time drawn must be written on label.
Sample must be centrifuged within two hours of collection. Serum/plasma (red or dark green top tube) must be removed from the red cells and put into aliquot tubes following centrifugation.

Transport Temperature

Refrigerated

Specimen Stability

Unstable, centrifuge within two hours of collection
Ambient: 2 days
Refrigerated: 8 days
Frozen: 5 months

Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)

Samples drawn in a tube with a separator gel will be rejected. The separator gel may cause falsely lower results.

Methodology
CMIA, Chemiluminescent microparticle Assay

Setup Schedule

Sunday-Saturday

Reference Range

Therapeutic level: 1.0-2.0 ug/mL
Toxic level: ≥2.1 ug/mL

Clinical Significance

Assessing compliance or toxicity of phenytoin treatment in patients with hypoalbuminemia or severe renal disease.

Total phenytoin levels may underestimate free drug concentrations in patients with low albumin or severe renal disease. Consider adjusting total phenytoin level based on albumin levels and creatinine clearance, or free phenytoin testing for these patients.

Performing Laboratory
West Virginia University Hospital, Inc.

The CPT Codes provided in this document are based on AMA guidelines and are for informational purposes only. CPT coding is the sole responsibility of the billing party. Please direct any questions regarding coding to the payor being billed. Any Profile/panel component may be ordered separately. Reflex tests are performed at an additional charge.