Specimen:

Plasma

Collection Medium:

Alternate
Collection Media:

Call laboratory for additional acceptable specimen collection 
containers.

Minimum:

2 ml; light green top tube or one 0.6 ml microtubes.  (Sufficient for 
one or more anticonvulsant drugs).

Testing
Schedule:

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

Analytic Time:

2 hours (upon receipt in laboratory)

Reference Range:

10-20 mcg/ml

Critical value: >40 mcg/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 immunoanalytic 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.

Please print, complete and submit the Advance Beneficiary Notice 
(ABN) along with the Laboratory Requisition before shipping the 
specimen.

Test
Limitations:

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 
60 (approximate bilirubin concentration: 60 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 1000. (approximate triglycerides concentration: 2000 mg/dl). 
There is poor correlation between turbidity and triglycerides 
concentration.

No significant interference from total protein up to 12 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. If this occurs, contact Customer Technical Support.

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:

EIA (Enzymatic ImmunoAssay)

Sample
Processing:

Centrifuge at 3000 RPM for 10 minutes.
Label transport tube with two patient identifiers, date and time of 
collection.

Sample
Storage:

Refrigerate.

Transport
Instructions:

Place requisition into outside pocket of bag.
Place specimen into zip-lock type bag, seal bag.
Transport in cooler with refrigerated coolant packs.

Instructions:

Add-on testing permitted only within 24 hours of draw time.

CPT Code:

80185

See also:
Phenytoin, Free, Plasma

See Additional Information:
Chemistry Critical Lab Values