Case: 52 year old home bound male with chronic history of seizure presented with hyperreflexia, confusion and nystagmus. You are worried about phenytoin toxicity but level is reported as 18 mcg/mL (normal is 10-20 mcg/mL). You secure ABC (airway, breathing and circulation). You start treating patient with IV Lorazepam. What should be your next step?
Answer: Check free phenytoin level
Its true that the therapeutic range of phenytoin is 10-20 mcg/mL and plasma levels (mcg/mL) have an association with acute neurological symptoms. In individuals with decreased protein binding - nursing home residents, poor diet, chronically ill etc - may have signs of toxicity despite a normal total phenytoin level. Free phenytoin levels should be checked. Free phenytoin levels range from 1-2 mcg/mL and correlate well with clinical evidence of toxicity.
If free phenytoin level is not available, it should be adjusted with albumin level with following formula, called Sheiner-Tozer equation. Its not 100% accurate but give good estimate.
Corrected Dilantin = measured level / [ (0.2 x albumin) + 0.1]
e.g: if measured Dilantin level is 8.2 but albumin is 2.2, the corrected Dilantin level would be 8.2 / { (.2 x 2.2) + .1} = 15.2
In renal patients, If patient CrCl is less than 20, use following formula.
Corrected Dilantin = measured level / [ (0.1 x albumin) + 0.1]
e.g: if measured Dilantin level is 8.2 and albumin is 2.2, the corrected Dilantin level would be 8.2 / { (.1 x 2.2) + .1} = 25.6
