Phenytoin, Free
Use
Free phenytoin measurement is clinically useful for precise therapeutic monitoring — especially in conditions that affect phenytoin protein binding such as uremia, hypoalbuminemia, hepatic impairment, late pregnancy, or polypharmacy — because only unbound drug is pharmacologically active and correlates better with clinical response than total drug levels ([jdos.nicholsinstitute.com](https://jdos.nicholsinstitute.com/dos/fmh/test/180068?utm_source=openai)).
Special Instructions
Collect as a trough level (just prior to next phenytoin dose); for fosphenytoin therapy, collect as a peak (at least 2 hours after IV infusion or at least 4 hours after IM injection) ([jdos.nicholsinstitute.com](https://jdos.nicholsinstitute.com/dos/fmh/test/180068?utm_source=openai)).
Limitations
Not provided.
Methodology
Immunoassay (Multiplex Protein Panel)
Biomarkers
Result Turnaround Time
1 day
Related Documents
For more information, please review the documents below
Specimen
Serum
Volume
2.5 mL
Minimum Volume
1 mL
Container
red-top tube (no gel)
Collection Instructions
Collect as a trough just prior to next dose; for fosphenytoin therapy collect as peak (at least 2 h after IV or at least 4 h after IM)
Causes for Rejection
Serum separator tube (SST), Plasma
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 4 days |
| Refrigerated | 7 days |
| Frozen | 30 days |