1.0-1.9 mcg/mL Average percent free is 9.6% with a range of 7.0-14.6% (N=91).

Note: Phenytoin plasma specimens should not be drawn for 2 hours following completion of an I.V. fosphenytoin infusion or 4 hours following an I.M. fosphenytoin dose (See Fosphenytoin). To accurately determine the percentage of free phenytoin, a total phenytoin should be ordered on the same sample. Please print, complete and submit the Advance Beneficiary Notice (ABN) along with the Laboratory Requisition before shipping the specimen.

Criterion: recovery plus or minus 0.6 ug/mL of initial value at concentrations less than 6 ug/mL or plus or minus 10% of initial value at concentrations greater than 6 ug/mL. Icterus: No significant interference from bilirubin up to an I index of 30 (approximate bilirubin concentration: 60 mg/dL). Hemolysis: No significant interference from hemoglobin up to an H index of 800 (approximate hemoglobin concentration: 1000 mg/dL). Lipemia (Intralipid): No significant interference from lipemia up to an L index of 750. (approximate triglycerides concentration: 2000 mg/dL). There is poor correlation between turbidity and triglycerides concentration.