Phenytoin, Free, Serum or Plasma

Phenytoin, Free, Serum or Plasma

CPT Test code: 80186

Specimen: Serum or plasma
Volume: 3 mL
Minimum Volume: 1.5 mL
Container: Red-top tube, lavender-top (EDTA) tube, or green-top (heparin) tube. Do not use a gel-barrier tube. The use of gel-barrier tubes is not recommended due to slow absorption of the drug by the gel. Depending on the specimen volume and storage time, the decrease in drug level due to absorption may be clinically significant.
Collection: Transfer separated serum or plasma to a plastic transport tube. Draw specimen just prior to next dose.
Storage Instructions: Refrigerate
Causes for Rejection: Gel-barrier tube; hemolysis; lipemia
Reference Interval: Therapeutic: 1.0-2.0 μg/mL
Additional Information: Phenytoin is 90% bound, but only the free fraction circulates through plasma membranes and is biologically active. Because of rapid equilibration between free and bound portions of drugs, free levels are potentially important only in AEDs that are highly bound (ie, phenytoin but not carbamazepine). Measurement of the free fraction is not cost-effective on a routine outpatient basis, but may be clinically relevant in exceptional circumstances associated with alterations in the binding of phenytoin.1 Binding kinetics may be altered in uremia, hepatic disease, late pregnancy or postpartum, cases of head injury associated with a hypermetabolic state, and certain instances of polypharmacy, described below.2-4 Determination of free levels may also be helpful in overdosages, since only the free portion can be cleared by dialysis.

In renal disease, total phenytoin levels may generate falsely high values, leading to inadequate dosage. Dialysis may increase the amount of free phenytoin available.5 In hepatic disease, phenytoin competes with endogenous bilirubin for binding sites,6 and thus the need for a free level may be greatest if the total bilirubin level is high and albumin low. Available liver function tests are not predictive of free phenytoin levels in patients with liver disease.4

Drugs which compete for binding sites on albumin and which may displace phenytoin include valproic acid, acetazolamide, high doses of salicylic acid, phenylbutazone, ceftriaxone, nafcillin, and sulfamethoxazole.7 In a clinical setting in which one of these drugs is used with phenytoin and toxicity is suspected despite normal phenytoin levels, a free level may be useful.

The free phenytoin level can be approximated by the total phenytoin level in cerebrospinal fluid or saliva or other body fluids that are albumin-poor.

Footnotes: 1. Theodore WH, Yu L, Price B, et al, “The Clinical Value of Free Phenytoin Levels,” Ann Neurol, 1985, 18(1):90-3. PubMed 4037755

2. Levy RH, Mattson RH, Maldrum BS, et al, Antiepileptic Drugs, 5th ed, Philadelphia, PA: Lippincott Williams and WIlkins, 2002.

3. Griebel ML, Kearns GL, Fiser DH, et al, “Phenytoin Protein Binding in Pediatric Patients With Acute Traumatic Injury,” Crit Care Med, 1990, 18(4):385-91. PubMed 1969339

4. Dasgupta A, Dennen DA, Dean R, et al, “Prediction of Free Phenytoin Levels Based on Total Phenytoin:Albumin Ratios,” Am J Clin Pathol, 1991, 95(2):253-6. PubMed 1992618

5. Dasgupta A, Abu-Alfa A, “Increased Free Phenytoin Concentrations in Predialysis Serum Compared to Postdialysis Serum in Patients With Uremia Treated With Hemodialysis: Role of Uremic Compounds,” Am J Clin Pathol, 1992, 98(1):19-25. PubMed 1615921

6. Hooper WD, Bochner F, Eadie MJ, et al, “Plasma Protein Binding of Diphenylhydantoin. Effects of Sex Hormones, Renal and Hepatic Disease,” Clin Pharmacol Therapeutics, 1974, 15(3):276-82. PubMed 4815953

7. Dasgupta A, Dennen DA, Dean R, et al, “Displacement of Phenytoin From Serum Protein Carriers by Antibiotics: Studies With Ceftriaxone, Nafcillin, and Sulfamethoxazole,” Clin Chem, 1991, 37(1):98-100.PubMed 1988217