Phenytoin

Label Mnemonic:	DPH
Epic code:	LAB31
Downtime form:	Doctor/Provider Orders - Pathology Core and Specialty Care Nursery

Chemistry
6240 RCP
356-3527

Specimen(s):

Plasma

Collection Medium:

Plasma Separator Tube 4.5 mL

Alternate Collection Media:

Call laboratory for additional acceptable specimen collection containers.

Minimum:

3 mL; light green top tube or ONE Microtainer^®.

Testing Schedule:

24 hrs/day, 7 days a week, including holidays.

Turn Around Time:

1 hour (upon receipt in laboratory)

Reference Range:

10-20 μg/mL

Critical value: >40 μg/mL

Comments:

Fosphenytoin is a pro-drug that is metabolized to phenytoin. The inactive pro-drug is cross-reactive with serum phenytoin when measured by immuno analytic assays. To avoid significant false elevations in serum phenytoin or serum free phenytoin, phenytoin serum 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.

Test Limitations:

Criterion: recovery within plus or minus 0.6 g/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.

Icterus: No significant interference from bilirubin up to an I index 
of 50 (approximate bilirubin concentration: 50 mg/dL).

Hemolysis: No significant interference from hemoglobin up to an H 
index of 1000 (approximate hemoglobin concentration: 1000 mg/dL).

Lipemia (Intralipid): No significant interference from lipemia up to 
an L index of 800. There is poor correlation between turbidity and 
triglycerides concentration.

No significant interference from total protein up to 14 g/dL.

No significant interference from rheumatoid factors up to 100 IU/mL.

The incidence of patients with antibodies to E. coli beta-
galactosidase is extremely low. However, some samples containing such 
antibodies can result in artificially high phenytoin results that do 
not fit the clinical profile.

As with any assay employing mouse antibodies, the possibility exists 
for interference by human anti-mouse antibodies (HAMA) in the sample, 
which could cause falsely elevated results.

Methodology:

Kinetic Interaction of Microparticles in Solution (KIMS)

CPT Code:

80185