PHENYTOIN,FREE

Code
900.3470
Name
PHENYTOIN,FREE
Category
None
Department
Send-Out
Start Date
Expiration Date
Synonyms
Dilantin
CPT Codes
80186
Site
SBMF
Reference Test
23130
ATLAS Test Code

Specimen Information

Type

Red, Plain

Volume

1.5 ml

Transport Info

Centrifuge and immediately transfer serum to separate plastic tube
Frozen

Fasting Required?
False
Patient Instructions

Reference Range

1.0-2.0 µg/mL
3.00 µg/mL

Methodology

Fluorescence Polarization

Clinical Significance

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
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.

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