Phenytoin TDM Flashcards
What types of seizures is phenytoin used for?
Tonic-clonic
Focal
Is phenytoin an enzyme inducer or inhibitor?
Inducer, so the affected drug dose needs to be increased
What is the therapeutic range for phenytoin?
10-20mg/L
What protein does phenytoin bind to, what form of phenytoin is the active form?
It binds to albumin, and only free unbound phenytoin is the active form
What can hypoalbuminaemia cause in relation to phenytoin?
It can cause an increase in the free active form of phenytoin, leading to toxicity
What are the side effects of phenytoin?
Agranulocytosis
Blood/skin disorders
Slurred speech
Ataxia
Peripheral neuropathy
Increased risk of fractures due to enhanced vit. D metabolism
What 2 parameters do you always measure when checking serum phenytoin levels?
Albumin concentration
Renal function
Which drugs increase serum phenytoin levels?
Enzyme inhibitors like -
Amiodarone
Antifungals - miconazole, ketoconazole, fluconazole
Chloramphenicol
Acute alcohol
Omeprazole
Nifedipine
Diltiazem
What drugs decrease serum phenytoin levels?
Enzyme inducers like -
Chronic alcohol
Smoking
Rifampicin
St Johns wort
What should you monitor when using phenytoin?
ADRs
LFTs
FBC
Blood/skin disorders
What are the signs of phenytoin overdose?
Ataxia
Respiratory depression
Involuntary eye movements - nystagmus
Difficulty speaking
When should you check serum phenytoin levels after any dose changes?
14 days after
