Phenytoin has significant protein binding, so hypoalbuminemia will lead to increased free phenytoin.
Adjust for hypoalbuminemia using the following formula = Total Phenytoin Level / [(0.2*albumin)+0.1]
*0.2 is a correction factor
In renal failure, aside from hypoalbuminia leading to increased free phenytoin, phenytoin binding is further reduced, and this formula will tend to underestimate the free phenytoin levels. To correct for this, use 0.1 as correction factor if GFR is <10 ml/min.
Drug monitoring: (http://emedicine.medscape.com/article/2090306-overview#a2)
- loading doses should be checked 1 hour after IV load and 24 hours after oral loading
- long term phenytoin therapy generally do not need to be monitored at intervals <3-12 months after steady state reached unless clinically indicated
- reference range is 10-20 ug/mL – although half of patients’ seizures are controlled at values lower and higher than therapeutic range
- adverse effects related to serum levels
- nystagmus if >20 ug/mL
- slurreg speech, movement disorders (tremor, choreoathetosis, orofacial dyskinesia) if >30 ug/mL
- lethargy, stupor, confusion if >40 ug/mL