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Phenytoin, Free [3189X]
Test Code23692
CPT Codes
80186
Preferred Specimen
2.5 mL serum collected in a red-top tube (no gel)
Patient Preparation
Collect as a trough just prior to next dose. For patients receiving fosphenytoin therapy, collect as peak (at least 2 hours after IV infusion or at least 4 hours after IM injection).
Minimum Volume
1 mL
Transport Container
Plastic screw-cap vial
Transport Temperature
Room temperature
Specimen Stability
Room temperature: 4 days
Refrigerated: 7 days
Frozen: 30 days
Refrigerated: 7 days
Frozen: 30 days
Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)
Specimen received in serum separator tube (SST) • Plasma
Methodology
Ultrafiltration/Immunoassay
Setup Schedule
Set up: Mon-Sat a.m.; Report available: Next day
Reference Range
1.0-2.0 mg/L
Clinical Significance
Phenytoin is used singly or in combination with other anticonvulsants to treat grand mal epilepsy. Monitoring the serum levels of antiepileptic drugs has increased the efficiency and safety of drug therapy in epilepsy. It facilitates individualization of dosage regimen, reveals irregular drug intake and identifies the responsible agent in intoxicated patients on multiple drug therapy. Free phenytoin is especially useful in patients with altered phenytoin binding to protein, e.g., uremia or individuals having abnormally low levels of albumin, such as nephrotic syndrome. Free phenytoin in serum correlates better with the various other free compartments of the body than does the total drug and is probably a better estimate of the "active drug level" at the receptor site and agrees more with the clinical response.
Performing Laboratory
Quest Diagnostics Nichols Institute
14225 Newbrook Drive
Chantilly, VA 20153
