Phenytoin

Hormone/ Element

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No Fasting Required

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Measures the concentration of phenytoin in the blood

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Phenytoin Test Information Guide

Why is it done?
- Measures blood levels of phenytoin, an anticonvulsant medication used to prevent and treat seizures
- Monitors therapeutic drug levels to ensure seizure control without toxicity
- Detects toxic levels that may cause adverse effects or drug toxicity
- Performed when initiating phenytoin therapy to establish baseline levels
- Used when adjusting dosages or when patient shows signs of inadequate seizure control
- Ordered when symptoms of toxicity appear such as dizziness, ataxia, or nystagmus
- Typically performed 5-7 days after starting therapy to allow steady-state levels (usual half-life 24 hours)
Normal Range
- Therapeutic Range: 10-20 mcg/mL (40-80 µmol/L)
- Toxic Level: >20 mcg/mL (>80 µmol/L)
- Subtherapeutic Level: <10 mcg/mL (<40 µmol/L)
- Units of Measurement: Micrograms per milliliter (mcg/mL) or micromoles per liter (µmol/L)
- Interpretation: Values within the therapeutic range indicate adequate seizure control with minimal risk of adverse effects. Levels below therapeutic range may result in breakthrough seizures. Levels above therapeutic range indicate risk of phenytoin toxicity and dose reduction may be needed.
- Note: Some patients may require levels up to 20-25 mcg/mL for optimal seizure control, while others experience toxicity at lower levels
Interpretation
- Therapeutic Range (10-20 mcg/mL): Optimal seizure control with acceptable side effects. Continue current dose regimen and monitor clinically.
- Subtherapeutic (<10 mcg/mL): May be inadequate for seizure control. Consider dose increase if breakthrough seizures occur. Poor compliance, rapid metabolism, or drug interactions may contribute.
- Toxic (>20 mcg/mL): High risk of adverse effects and phenytoin toxicity. Dose reduction is necessary. Symptoms of toxicity may be present.
- Free Phenytoin Level: Therapeutic range 1-2.5 mcg/mL. Unbound phenytoin is pharmacologically active and may be clinically more accurate in certain populations.
- Factors Affecting Results: Hypoalbuminemia (increases free active drug), hepatic disease, renal disease, pregnancy, drug interactions, malnutrition, and genetic variations in metabolism
- Symptoms of Toxicity: Ataxia, nystagmus, diplopia, dizziness, confusion, hyperglycemia, gum hyperplasia, hirsutism, and tremor
Associated Organs
- Primary Organ Systems: Central nervous system (CNS), liver, kidneys, and blood
- Seizure Disorders: Generalized tonic-clonic seizures, partial seizures, status epilepticus
- Cardiovascular: Used for cardiac arrhythmia management in some cases; toxicity may cause hypotension and cardiac conduction abnormalities
- Hepatic Complications: Phenytoin is metabolized primarily in the liver; hepatic disease increases toxicity risk
- Hematologic Disorders: Phenytoin toxicity associated with megaloblastic anemia, thrombocytopenia, and leukopenia
- Skin Reactions: Stevens-Johnson syndrome, DRESS syndrome, and other severe hypersensitivity reactions may occur
- Metabolic Effects: Can cause hyperglycemia, folate deficiency, bone disease, and vitamin K deficiency
Follow-up Tests
- Repeat Phenytoin Level: Performed 5-7 days after dose adjustment to assess new steady-state level
- Free Phenytoin Level: Ordered if total level is in therapeutic range but patient has signs of toxicity, or in patients with hypoalbuminemia
- Liver Function Tests (LFTs): Assess hepatic metabolism capacity and potential drug-induced liver injury
- Complete Blood Count (CBC): Monitor for bone marrow suppression and hematologic complications
- Kidney Function Tests: Creatinine and BUN to assess renal function; particularly important in elderly or compromised patients
- Serum Albumin: Assess protein binding; influences interpretation of total phenytoin level
- Glucose Testing: Monitor for hyperglycemia, a known side effect of phenytoin
- Monitoring Frequency: Initially every 5-7 days during initiation, then monthly for 2 months, then every 6-12 months for maintenance therapy
- EEG Monitoring: Assess adequacy of seizure control in patients with ongoing breakthrough seizures
Fasting Required?
- Fasting Requirement: No - Fasting is NOT required for phenytoin level testing
- Timing of Sample Collection: Draw trough samples (just before next scheduled dose) for most accurate therapeutic monitoring. Can be drawn anytime for steady-state assessment after 5-7 days of therapy.
- Medications to Avoid: Do not discontinue phenytoin before test. Continue all regular medications as prescribed. Inform healthcare provider of all medications, supplements, and herbal products being taken.
- Patient Preparation: No special preparation needed. Patient may eat and drink normally. Blood sample is typically drawn via venipuncture into a serum separator tube.
- Special Instructions: Maintain consistent dosing schedule before test. Document exact time of last dose and time of blood draw for accurate interpretation. Inform phlebotomist if patient is pregnant, as phenytoin use requires special consideration.

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