Phenytoin, Free and Total
0090141
Ordering Recommendation
Preferred test for therapeutic drug management in patients with renal failure or conditions that may alter albumin concentrations.
Mnemonic
FDIL
Methodology
Quantitative Enzyme Multiplied Immunoassay Technique
Performed
Tue-Sat
Reported
1-4 days  
New York DOH Approval Status
This test is New York DOH approved.
Submit With Order
Specimen Required
Patient Preparation
Timing of specimen collection: Pre-dose (trough) draw - At steady state concentration.  
Collect
Plain red.  
Specimen Preparation
Separate serum from cells within 2 hours of collection. Transfer 2 mL serum to an ARUP Standard Transport Tube. (Min: 1 mL)  
Storage/Transport Temperature
Refrigerated.  
Unacceptable Conditions
Citrated plasma. Serum separator tubes (SST). Tubes that contain liquid anticoagulant.  
Remarks
  
Stability
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 Phenytoin -​ Total Therapeutic: 10.0-​20.0 µg/mL
Toxic: > 30.0 µg/mL
No Phenytoin -​ Free Level Therapeutic: 1.0-​2.0 µg/mL
Toxic: > 3.0 µg/mL
No Phenytoin -​ Percent Free 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 percent) 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
Statement B: This test was developed and its performance characteristics determined by ARUP Laboratories. The U.S. Food and Drug Administration has not approved or cleared this test; however, FDA clearance or approval is not currently required for clinical use. The results are not intended to be used as the sole means for clinical diagnosis or patient management decisions.
 
Note
 
CPT Code(s)
80185; 80186
Components
Component Test Code*Component Chart NameLOINC
0090125Phenytoin - Free Level3969-3
0090220Phenytoin - Percent Free10548-6
0090280Phenytoin - Total3968-5
* Component test codes cannot be used to order tests. The information provided here is not sufficient for interface builds; for a complete test mix, please click the sidebar link to access the Interface Map.
Aliases
  • Dilantin, Free
  • Dilantin, Total and Free
  • Diphenylan
  • Diphenylhydantoin
  • Fosphenytoin
  • Free Dilantin
  • Free Phenytoin
  • Phenytek
  • pheyntoin blood concentration
  • Protein Free Phenytoin