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Homogeneous Immunoassays—Therapeutic Drug Monitoring
QMS®—Latex Enhanced Immunoturbidimetric Assays
Phenytoin Assay System
The PHENYTOIN Assay System is intended for the quantitative determination of total phenytoin in serum for therapeutic drug monitoring.
Phenytoin is an anticonvulsant drug used to treat all forms of epilepsy except absence seizures. Phenytoin acts directly on the central nervous system to block seizure activity.1-3
Phenytoin absorption following oral dosing is variable.2,3 Peak plasma concentrations may occur from as early as three hours to as late as twelve hours following drug administration.2 Once absorbed, phenytoin is rapidly distributed into all tissues. The half-life of phenytoin ranges from 20 to 60 hours at therapeutic concentrations. Phenytoin is bound extensively (about 90%0 to plasma proteins, mainly albumin.2
Metabolism of phenytoin occurs primarily in the liver. The major metabolite, the parahydroxyphenyl derivative, is apparently inactive.2-5 Phenytoin exhibits dose-dependent metabolism and at higher serum concentrations small dosage changes can result in large serum concentration changes. Approximately 2-5% of the drug is excreted unchanged in the urine.2 In addition, concomitant medications may increase or decrease serum phenytoin levels.2,3,6 Monitoring serum phenytoin concentrations is recommended to guide dosage and avoid toxicity effects due to patient differences in the absorption, metabolism and clearance of phenytoin.2,3,6,7 Immunoassays for phenytoin using a fluorescence polarization immunoassay have been described.8,9
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