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Phenytoin, Free
MessagePerforming Lab: Regions
Test Code
3581
Alias/See Also
Sunquest: FPHEN; Free Dilantin
CPT Codes
80186
Includes
Time of last dose, Free Phenytoin
Preferred Specimen
0.5 mL Lithium Heparin Plasma (light green-top) tube
Minimum Volume
0.2 mL
Other Acceptable Specimens
Serum separator (gold-top), Lithium Heparin (light green-top) on ICE, Red, Red/Gray
Transport Temperature
Refrigerated
Specimen Stability
Room Temperature: 7 days
Refrigerated: 7 days
Frozen: 1 month
Refrigerated: 7 days
Frozen: 1 month
Methodology
Enzyme Immunoassay (EIA)
Setup Schedule
Daily
Report Available
Same day.
Reference Range
5 - 10% of the Total Phenytoin
Clinical Significance
The pharmacologic effect of anticonvulsant drugs is directly related to the amount of drug present in the free or unbound state. Only the free portion is capable of crossing biologic membranes and interacting at important binding sites. Since the degree of protein binding can be affected by the presence of other drugs and altered physiological states, free drug determinations represent an important tool for customizing individual patient anticonvulsant levels.
Phenytoin is one of the most widely prescribed drugs for the treatment of seizure disorders. It is highly effective in the control of grand mal, focal sensory and motor, and psychomotor seizures, but of no use in petit mal seizures. There is a strong correlation between serum levels and both therapeutic and toxic effects. In most cases, seizure control is obtained with serum levels between 1 µg/mL and 20 µg/mL and toxic effects are usually associated with serum levels above 25 µg/mL. The main pathway for phenytoin elimination is a saturable process that could lead to toxic serum levels with a small increase in dosage. Because both metabolism and clearance are variable amounts, close monitoring of the serum levels of phenytoin in patients is necessary.
Phenytoin is one of the most widely prescribed drugs for the treatment of seizure disorders. It is highly effective in the control of grand mal, focal sensory and motor, and psychomotor seizures, but of no use in petit mal seizures. There is a strong correlation between serum levels and both therapeutic and toxic effects. In most cases, seizure control is obtained with serum levels between 1 µg/mL and 20 µg/mL and toxic effects are usually associated with serum levels above 25 µg/mL. The main pathway for phenytoin elimination is a saturable process that could lead to toxic serum levels with a small increase in dosage. Because both metabolism and clearance are variable amounts, close monitoring of the serum levels of phenytoin in patients is necessary.
