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Phenytoin, Free

Item Value
Approval req'd? No
Available Stat? No
Test code PHNYF
Test group Phenytoin
Performed by Quest
Sendout? Yes
Method Ultrafiltration-High Pressure Liquid Chromatography
Collection Instructions Time to steady state: 3-4 days

Collect samples 30 minutes before AM dose.

Draw at least 4 hours post-IV dose and 6-9 hours post-PO dose

Collect in Red top tube only. Avoid hemolysis.

Order total phenytoin separately if desired. Indicate time of collection on requiisition.
Container type Red top (Gold top NOT acceptable)
Amount to Collect 5 mL blood
Sample type Serum
Preferred volume 2.5 mL serum
Min. Volume 1.5 mL serum
UCSF Rejection Criteria Collected in Gold top. Gross hemolysis
Processing notes Refrigerate separated serum. Be sure that total Phenytoin has been separately ordered, if requested. Order Quest # 39693P

For B&T patients order labCorp test #070706 "Phenytoin Free & Total only"
Units µg/mL (mcg/mL)
Normal range
Therapeutic 1.0-2.0 µg/mL
Potentially toxic > 3.0 µg/mL
Synonyms Dilantin; Fosphenytoin; Cerebyx
Stability Room temperature 4 days, refrigerated 1 week, frozen at -20C 1 month
Turn around times Run 5x per week. Turnaround 3-5 days
Additional information Phenytoin binds to albumin, only the free fraction is therapeutically active. In patients with hypoalbuminemia, who are on dialysis, are critically ill or are receiving medications that may displace phenytoin the measurement of total phenytoin levels may result in either under or overdosing. In these situations assessing free phenytoin is suggested.
CPT coding 80186-90
LOINC code 3969-3
Last Updated 10/28/2013 11:49:04 AM
Entry Number 1099
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