Phenytoin, Serum or Plasma

CPT: 80185
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  • Dilantin®
  • Diphenylhydantoin
  • Phenytek®

Expected Turnaround Time

Within 1 day

Related Documents

Specimen Requirements


Serum or plasma


1 mL

Minimum Volume

0.8 mL


Red-top tube or green-top (heparin) tube. Do not use a gel-barrier tube. The use of gel-barrier tubes is not recommended due to slow absorption of the drug by the gel. Depending on the specimen volume and storage time, the decrease in drug level due to absorption may be clinically significant.


Transfer separated serum or plasma to a plastic transport tube. IV: two to four hours after loading dose. Oral: peak: three to nine hours after dose, immediately prior to next dose. Optimal resampling after change in dosage: 48 hours.

Storage Instructions

Room temperature

Stability Requirements



Room temperature

14 days


14 days


14 days

Freeze/thaw cycles

Stable x3

Causes for Rejection

Gel-barrier tube; severe hemolysis; lipemia

Test Details


Phenytoin is useful in generalized tonic-clonic, complex partial, and simple partial seizures and frequently is chosen for initial therapy, particularly in adults. Because of phenytoin's potential adverse reactions (hirsutism, gingival hyperplasia, coarsening of facial features), other antiepileptic drugs are often prescribed for infants and young children. Phenytoin is commonly given with phenobarbital, primidone, carbamazepine, or valproate when monotherapy fails. It is ineffective in absence, myoclonic, and atonic seizures and is not recommended for the treatment of infantile spasms, Lennox-Gastaut syndrome, and epileptic syndromes in older children and adolescents when absence seizures or myoclonus is present. Intravenous phenytoin sodium is effective for status epilepticus and can be used as the initial drug to manage recurrent seizures if they are widely spaced. This drug also may prevent seizures initially in high-risk patients with head trauma. Phenytoin has been advocated for many other disorders, but conclusive evidence of effectiveness is inadequate for most proposed indications.



Reference Interval


• <90 days: 6.0−14.0 μg/mL

• Adults: 10.0−20.0 μg/mL

Critical Value

Potentially toxic: Adults: >20.0 μg/mL

Additional Information

As a potent inducer of P450 enzymes CYP3A4, CYP1A2, and CYP2C9, co-administration of phenytoin can decrease plasma concentrations of tricyclic antidepressants and most other antiepileptic drugs.1,2 Despite normal levels, phenytoin may interfere with the actions of other drugs, including cyclosporine, oral anticoagulants, oral contraceptives, and theophylline; appropriate laboratory monitoring of some of these agents is advised. The half-life of phenytoin in adults is 20 to 40 hours; in children, it is approximately 10 hours. Teratogenic effects of phenytoin have been proposed but not confirmed; the risks in women of childbearing age should be balanced against the risks of increased seizures.3


1. Spina E, Perucca E. Clinical significance of pharmacokinetic interactions between antiepileptic and psychotropic drugs. Epilepsia. 2002; 43(Suppl 2):37-44. 11903482
2. Bourgeois BF. Pharmacokinetic properties of current antiepileptic drugs. What improvements are needed? Neurology. 2000; 55(11Suppl 3):S11-S16. 11147563
3. Dalessio DJ. Current concepts: Seizure disorders and pregnancy. N Engl J Med. 1985 Feb 28; 312(9):559-563 (review). 3881676


American Medical Association, Division of Drugs and Toxicology. Drug Evaluations Subscription. Chicago, Ill: AMA, Fall 1992.


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
007401 Phenytoin (Dilantin), Serum 3968-5 007512 Phenytoin (Dilantin), Serum ug/mL 3968-5

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