Phenytoin, Free
Label Mnemonic: FDPH
Epic code: LAB32
Downtime form: A-1a Doctor/Provider Orders - Pathology Core and Specialty Care Nursery
6240 RCP
Collection Medium:
Plasma Separator Tube 4.5 mL
Alternate Collection Media:
Call laboratory for additional acceptable specimen collection containers.
3 mL whole blood from light green top tube or TWO Microtainer® devices.
Testing Schedule:
Daily. Sample must be in the lab by 1200; results available by 1600.
Turn Around Time:
24 hours (upon receipt in laboratory)
Reference Range:
1.0-1.9 μg/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.
Test Limitations:
Criterion: recovery plus or minus 0.6 μg/mL of initial value 
at concentrations less than 6 μg/mL or plus or minus 10% of 
initial value at concentrations greater than 6 μg/mL.
Homogeneous Enzyme Immunoassay
CPT Code:
See also:
Phenytoin, Plasma
See Additional Information:
Chemistry Critical Lab Values