ARUP's Laboratory Test Directory

Phenytoin, Free and Total : 0090141

Mnemonic: FDIL

Methodology: Quantitative Enzyme Multiplied Immunoassay Technique
Performed: Sun-Sat
Reported: 1-2 days
Specimen Required: Patient Prep: Timing of specimen collection: Pre-dose (trough) draw - At steady state concentration.

Collect: Plain red. Also acceptable (avoid if possible): Serum separator tube (follow preparation instructions below).

Specimen Preparation: Separate serum from cells within 2 hours of collection. Transfer 2 mL serum to an ARUP Standard Transport Tube. (Min: 1 mL)
SST: Serum in a gel separator tube stored at room temperature is acceptable if separated from the gel within 2 hours. Serum in a gel separator tube stored refrigerated is acceptable if separated from the gel within 1 hour.

Storage/Transport Temperature: Refrigerated.

Unacceptable Conditions: Citrated plasma. Tubes that contain liquid anticoagulant.

Stability (collection to initiation of testing): After separation from cells: Ambient: 4 days; Refrigerated: 4 days; Frozen: 1 month
Reference Interval:
Effective February 19, 2013
Available Separately Components Therapeutic Range
No Total Phenytoin Therapeutic: 10.0-20.0 µg/mL
Toxic: > 30.0 µg/mL
No Free Phenytoin Therapeutic: 1.0-2.0 µg/mL
Toxic: > 3.0 µg/mL
No Percent Free Phenytoin 8.0-14.0%

Interpretive Data: Free phenytoin may be important to monitor in patients with altered or unpredictable protein binding capacity because phenytoin is highly bound (greater than 90%) at therapeutic concentrations. Phenytoin is also subject to drug-drug interactions due to displacement of protein binding and extensive metabolism. Cross-reactivity with metabolites may account for differences in phenytoin concentrations among analytical methods. Calculating percent free attempts to minimize differences in assay cross-reactivity and may be useful in dose optimization.

See Compliance Statement B: www.aruplab.com/CS
CPT Code(s): 80185; 80186
Cross References: Dilantin, Free (Phenytoin, Free and Total), Diphenylan (Phenytoin, Free and Total), Diphenylhydantoin (Phenytoin, Free and Total), Fosphenytoin (Phenytoin, Free and Total), Free Dilantin (Phenytoin, Free and Total), Free Phenytoin (Phenytoin, Free and Total), Phenytek (Phenytoin, Free and Total), Protein Free Phenytoin (Phenytoin, Free and Total)