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Lab Test: Phenytoin, Total Level

    Lab Test
    • Phenytoin (Total)
    Description
    • Measurement of total phenytoin in serum or plasma for therapeutic drug monitoring and suspected toxicity
    Reference Range
    • Adults
      • Therapeutic level:  10-20 mcg/mL (40-79 micromol/L)
      • Toxic level:  >20 mcg/mL (>79 micromol/L)
      • Lethal level: > 100 mcg/mL (> 396 mmol/L)
    Indications & Uses
    • Suspected phenytoin toxicity
    • Therapeutic monitoring of phenytoin levels
    Clinical Application
    • Phenytoin is highly protein bound (mainly to albumin; 85-95%) and only the free (unbound) fraction is pharmacologically active.
    • Differences in formulation between brand names and generic equivalents can result in altered bioavailability, causing toxicity or the development of subtherapeutic levels.
    • Small changes in dosage can result in marked changes in blood levels.  The correct therapeutic dose of phenytoin should be determined by seizure control and clinical toxicity, not by serum drug levels.  Supratherapeutic levels are required to control seizures in some cases.
    • A serum phenytoin level is recommended several days after starting drug therapy, and again after 2 to 3 weeks to evaluate for delayed toxicity.
    • Toxic effects such as nystagmus, ataxia, and mental changes are usually seen at levels >20 mcg/mL, 30 mcg/mL, and 40 mcg/mL, respectively.  Signs and symptoms of toxicity may persist for up to 7 to 10 days.
    Related Tests
    • Albumin levels
    • Carbamazepine levels
    Drug-Lab Interactions
    • Clove will result in a false increase in phenytoin levels
    Test Tube Needed
    • Red top
    Procedure
    • Collect the level at least 2-4 hours after a loading dose when given by IV or 3-9 hours if dose given by mouth.
    • Trough levels collected right before the next dose
    Storage and Handling
    • Room temperature = 5 days
    • Refrigerated = up to 2 weeks
    • Frozen = up to 1 month
    What To Tell Patient Before & After
    • None
    References
    • Banh HL et al. Interpatient and intrapatient variability in phenytoin protein binding. Ther Drug Monit 2002;24:379085.
    • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.

MESH Terms & Keywords

  • Phenytoin, Total Level