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Lab Manual for UCSF Clinical Laboratories

Lab Manual for SFGH

Internal Resources


Item Value
Available Stat? Yes
Test code DPH
Performed by? Chemistry
Sendout? no
Price range $$$
In House Availability Routine: 7 days;
STAT: 24 hours/ 7 days
Principle The Siemens Centaur assay is a competitive immunoassay using a direct chemiluminescent technology.
Interpretation Subtherapeutic levels may require dosage adjustment.
> 20 mg/L (> 79 mcmol/L) nystagmus usually present
> 30 mg/L (> 119 mcmol/L) ataxia
> 40 mg/L (> 158 mcmol/L) Lethargy

Container type gold top gel tube
Amount to Collect 2 mL
Collection Instructions Plasma samples cannot be assayed
Sample type Blood
Normal range 10-20 mg/L (40-79 mcmol/L)

The therapeutic range for phenytoin is reduced in patients with low albumin (< 3.5 mg/dL) and those with severely impaired renal function (creatinine clearance < 10 mL/min). Contact inpatient pharmacy for more information (x8460).

Note: In hypoalbuminemic patients, the percentage of unbound phenytoin (normally 10%) increases by 1% for every 0.1 g/dL. In patients with low albumin but normal renal function, the following formula may be used to determine the normalized concentration (if patient had normal albumin). The "normalized concentration" may then be compared to the reference range (10-20 mg/L or 40-79 mcmol/L) for assessing the risk of toxicity.

Normalized Concentration = Observed concentration
0.9 ALB + 0.1

All values greater than 35 mg/L (> 139 mcmol/L) are considered Critical Values and are called to the physician or patient care unit immediately.
Synonyms Dilantin;
Stability 24 hours at 4°C.
Interferences 1. Increased drug metabolism may produce low levels. INH, valproate, phenobarbital, felbamate, coumadin, and antabuse compete with metabolism and give increased levels. Synergistic effects occur when phenytoin is used with carbamazepine or valproic acid.

2. Phenytoin is highly protein bound (@ 90%), primarily to albumin.

Factors that alter protein binding, such as

* low serum albumin (< 3.5 mg/dL)

* renal failure

* jaundice

* competition from other drugs that bind to albumin (e.g. valproate),

can increase the free (unbound) concentration of phenytoin as well as phenytoin clearance. Patients with decreased protein binding of phenytoin may have good seizure control when serum phenytoin concentrations are subtherapeutic OR may have signs of toxicity/dose-related side effects when serum phenytoin is within the therapeutic range. Dosing may need to be adjusted. Contact the inpatient pharmacy (x68460) or the Clinical Chemistry Lab Medicine Resident (x65527 or pager 415-443-2311) for more information.

Phenytoin is extensively metabolized. Co-administration of other drugs can increase or decrease serum phenytoin concentrations. Well documented interactions include

* increased phenytoin levels with INH, valproate, cimetidine, oral anticoagulants, disulfiram

* decreased or variable effects with carbamazepine, phenobarbital, primidone.

3. Auto- (rheumatoid factor) and heterophile antibodies in human serum can react with reagent immunoglobulins, interfering with in vitro immunoassays. Patients routinely exposed to animals or to animal serum products can be prone to this interference. Anomalous values, spuriously low or high, may be observed. Results inconsistent with clinical findings or previous laboratory values can be investigated by the laboratory for this phenomenon. Call the Clinical Chemistry Laboratory Medicine Resident (x65527, pager 415-443-2311).

4. If IV Fosphenytoin is given, a phenytoin level should not be collected for 2 hours.
References 1. Penry, J.K. and Newmark, M.E.: Ann Int Med 90:207, 1979.

2. Bayer 119934 Rev. D, 2003-05. Siemens Medical Solutions Diagnostics, Malvern, PA.

3. Basic Clinical Pharmacokinetics. 2nd Edition, M.E. Winter, Chapter 9, Applied Therapeutics, Spokane Wash., 1988.
CPT coding 80185
Last Updated 6/18/2011 8:27:32 AM
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