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ID:158 Phenytoin
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DescriptionAn anticonvusant drug. Phenytoin is strongly bound to albumin and plasma concentrations should be interpreted in the light of plasma albumin. PLEASE NOTE: TEST CAN ONLY BE ADDED TO A SAMPLE WITHIN 8 HOURS OF COLLECTION
IndicationTherapeutic Drug Monitoring (TDM).
Additional InfoFor monitoring purposes blood should not be drawn before a steady state is attained. This will not be achieved for at least 7 days after starting therapy or changing dose. In children, this period is slightly shorter at 5 days. Small changes in dose can also affect serum levels. PHARMACOKINETICS: Phenytoin is an anti epileptic drug (note it is not effective for absence seizures). Its mechanism of action is modulated by Na,K ATPase which results in blockage of neurotransmitter release. Phenytoin is not readily soluble in aqueous solution and if administered IV it tends to precipitate at the site of injection and is slowly absorbed. The oral absorption of phenytoin is also slow and sometime incomplete. However, once absorbed the drug is highly protein bound (90 - 95%). The pharmacological effect of phenytoin is directly related to the amount of drug present in the free (unbound) state as only free phenytoin is able to cross biological membranes and interact with the biologically important binding site. Side effects of therapy include nausea, vomiting, ataxia, slurred speech. The hepatic metabolism of phenytoin can become saturated within the therapeutic range. Once saturated small dosage increments can result in large changes in blood concentration. The toxic effects of phenytoin include seizures, impaired consciousness and hypotension.
Concurrent Testsna
Dietary Requirementsna
InterpretationCautious adjustment of dose to therapeutic level is advised.

DepartmentBlood Sciences
TubeSerum or Heparin
Tube Picture
Collection ConditionsUse Gold top (serum gel) tube except for Intensive care units, renal unit, transplant unit, patients on IV heparin (use Green/Yellow top tube for these patients).
Min. Vol1 mL

Ref. Range (Male)5 - 20 mg/L
Ref. Range (Female)5 - 20 mg/L
Ref. Range (Paed)
Ref. Range NotesNOTE: Reporting units changed from mmol/L to mg/L May 1, 2007
IP Acute TATRefer to Website
IP Routine TATRefer to Website
GP Acute TAT- Contact Laboratory
GP Routine TATRefer to Website
Turnround CommentNA

Originally edited by : MC. Review due on 24/05/2017 07:59:35. Published By Sylvia McLellan on 24/05/2016 07:59:35.