Tuesday April 4, 2006
Phenytoin (Dilantin) level
The ideal phenytoin level is to have unbound (free) phenytoin level but if free phenytoin level is not available or turn around time is long, 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 but albumin is 2.2, the corrected Dilantin level would be 8.2 / { (.1 x 2.2) + .1} = 25.6
See the difference ?. Just don't carried away with low level.
Also be cautious, phenytoin's dose increase is not linearly related to serum levels. Small increase in dose may produce disproportionate and actually toxic serum level.
