Phenytoin, Free
Test Mnemonic
PHTFR
CPT Codes
- 80186 - QTY (1)
LOINC ®
3969-3
Aliases
- Dilantin, Free
Performing Laboratory
Cleveland Clinic Laboratories
Specimen Requirements
| Volume | Type | Container | Collect Temperature | Transport Temperature | Special Instructions |
|---|---|---|---|---|---|
| 2 mL | Serum | No additive (Red) | Refrigerated | Do 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 |
|---|---|
| Ambient | After separation from cells: 7 days |
| Refrigerated | After separation from cells: 7 days |
| Frozen | After 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 |
| Years | 99 Years | Normal | 1.0 - 2.0 | ug/mL | Years | 99 Years | Urgent | > 4.0 | ug/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.