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Test Name:
Phenytoin, Free
- SBMF No:
23130 - Performance Lab Name:
Automated Lab - Test Mnemonic:
DIL FREE - ABN:
Not required - CPT Code:
80186 - LOINC Code:
3969-3 - Also Known As:
Dilantin® - Also See:
31081 Phenytoin, Total - Spec Type:
Serum - Spec Container:
Red top (serum) tube - Alt Spec Type:
Plasma - Alt Spec Container:
Green top (lithium heparin) tube - Pref Vol:
1.5 mL - Min Vol:
1.0 mL - Fasting:
No - Spec Collect:
Do not use gel-barrier tubes for specimen collection
Routine venipuncture
Comparative measurements should have consistent sampling times
IV: Collect 24 hours after dose - Spec Process:
Refrigerate tube of blood and transport refrigerated on day collected
If specimen will not be transported to SBMF on day collected: - For samples collected in serum tubes, clot 30 minutes - Promptly centrifuge 15 minutes - Immediately transfer serum or plasma to a separate plastic tube and freeze (-20°C) - Spec Store Transport:
Refrigerated or Frozen - Spec Stability:
24 hours refrigerated (2-8°C)
Greater than 24 hours, freeze (-20°C) – Do not use frost-free units that undergo repeated freeze/thaw cycles - Methodology:
Fluorescence Polarization - Use:
When a patient's clinical response does not agree with the total phenytoin concentration, or the protein binding of a patient is believed to be abnormal, the free phenytoin level may correlate more accurately than the total level with the clinical effectiveness or toxicity of the drug - Clinical Significance:
Phenytoin (Dilantin®) is one of the most widely prescribed anticonvulsants and is occasionally used as a myocardial antiarrhythmic. In the treatment of epilepsy, phenytoin is indicated for grand mal epilepsy (major motor), cortical focal seizures and temporal lobe epilepsy.The total concentration of a drug in the blood consists of the protein-bound fraction and the free fraction of which only the free drug is considered to be pharmacologically active. Because phenytoin is highly bound to plasma proteins (89-95%), any alteration in the protein binding of phenytoin due to uremia, hypoalbuminemia, ingestion of other drugs or age can result in a significantly different clinical response to a total phenytoin concentration. When a patient's clinical response does not agree with the total phenytoin concentration, or the protein binding of a patient is believed to be abnormal, the free phenytoin level may correlate more accurately than the total level with the clinical effectiveness or toxicity of the drug. In such cases, the determination of the free level of phenytoin is essential.Strong correlations have been shown between free phenytoin levels and both therapeutic effect and toxicity. Clinical observations indicate that toxicity of phenytoin is increased in patients with abnormally low protein binding. Phenytoin protein binding is influenced by uremia, altered albumin concentrations, concurrent drug therapy, or age. Phenytoin toxicity primarily affects the central nervous system. Toxic levels can lead to nystagmus, vertigo, ataxia, psychoses and even convulsions. Chronic treatment leads to hyperplasia of gums, anemia and osteomalacia. The frequency and severity of dose-dependent toxic effects increase as the free level rises to or above 3.0 µg/mL. - Reference Range:
1.0-2.0 µg/mL - Critical High Value:
3.00 µg/mL - Day Run:
Sun-Sat (daily) - Time Run:
As received - Time Reported:
8 hours - Test Type:
TOX-TDM
