Procedure Logs

C3.1 R4(e): Perform & document a phenytoin pharmacokinetic interpretation

ID: 16 year old admitted secondary to trauma (motor vehicle accident); 70 kg; not actively seizing. Phenytoin started as prophylaxis of seizures

Two levels taken:

#1: 2 days post-admit (likely not at steady state)

PHT level = 29umol/L (N:40-80 umol/L)

Alb = 31 g/L

Corrected PHT = 33.14 umol/L

 

#2: 6 days post-admit (likely at steady state)

PHT level = 12 umol/L (N:40-80 umol/L)

Alb = 42 g/L

The first dose is likely not at steady state, so it only serves as a clinical “guider” of what to do.. e.g. if patient is seizing at the “29 umol/L dose” then increase dose. The second dose, however, can be useful with regards to some PK calculations!

Only have one dose at steady state, so must use some population parameters:

Population parameter for this age group (10-16) is:

Vmax: 8.3 mg/kg/day = 581mg/day, Km = 5.7mg/L, Vd = 0.7 L/kg = 49 L

S = 0.92, F= 1.0Screen Shot 2016-06-24 at 3.15.58 PMWith the rounded dose, my pt would still be in the therapeutic range!

I need to be mindful of thinking about all the possible alternatives feasible for this patient:

  1. D/C phenytoin
    • If so monitor for: presence of seizures
  2. Increase dose to 510mg/day (210mg QAM and 300mg QHS)
    • If so monitor for: presence of seizures, toxicity: nystagmus, slurred speech, ataxia (these might be masked by the brain injury), N/V –> difficult to assess as the patient is NPO; thus might have to solely look at arrhythmias and ECGs
  3. Option of mini-loading phenytoin then slowly tapering up to therapeutic levels

Patient has not had any seizures since trauma, no seizures while on subtherapeutic levels of phenytoin and there is minimal evidence for prophylaxis of seizures. Thus, I would suggest to D/C phenytoin.

I did this work over the weekend, so when I came back patient had already been discharged…and was discontinued off of phenytoin!

This served as a great exercise that while therapeutic levels and interpreting them are great, I need to be very mindful of the clinical “so what” of my findings are.

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