Phenytoin, Free




Test Mnemonic

PHTFR

CPT Codes

  • 80186 - QTY (1)

LOINC ®

3969-3

Aliases

  • Dilantin, Free

Performing Laboratory

n/a


Specimen Requirements

Volume Type Container Collect Temperature Transport Temperature Special Instructions
2 mLSerumNo additive (Red) RefrigeratedDo not collect in a gel separator tube. Centrifuge and transfer serum to a CCL tube and refrigerate. Do not share tube with another test.

Minimum Specimen Requirements

Volume Type Container Collect Temperature Transport Temperature Special Instructions
1.5 mL     

Stability

Environmental Condition Description
AmbientAfter separation from cells: 7 days
RefrigeratedAfter separation from cells: 7 days
FrozenAfter separation from cells: 16 weeks

Days Performed

Sun - Sat

Turnaround Time

8 hours

Methodology

Name Description
Kinetic Interaction of Microparticles in a Solution 

Reference Range

Phenytoin, Free
Sex Age From Age To Type Range Range Unit
   Years99 YearsNormal1.0 - 2.0ug/mL
   Years99 YearsUrgent> 4.0ug/mL

Special Info

Do not collect in a gel separator tube. Draw once steady state is achieved. Usual sampling time varies dependent upon desired measurement of peak or trough.

Clinical Info

Phenytoin is highly protein bound, and only the free (unbound) fraction is pharmacologically active. The free fraction may be increased, for instance, due to interaction with other highly protein-bound drugs, or in patients with albumin deficiency, anemia, uremia or with hepatic or renal insufficiency, resulting in an increased effect at the same total drug concentration as in plasma of normal patients. Monitoring the free phenytoin concentration can be useful in response to these altered disposition states, especially, in case the clinical picture of the patient does not correlate well with the total serum/plasma phenytoin concentration.

Clinical Limitation

Due to the observed cross-reactivity of the assay to fosphenytoin, it is recommended that samples for serum phenytoin measurements be collected at least 2 hours after an intravenous dose of fosphenytoin and at least 4 hours after an intramuscular dose.