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Lab Manual for UCSF Clinical Laboratories

Lab Manual for SFGH

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PHENOBARBITAL

Item Value
Available Stat? Yes
Test code PBAR
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 direct chemiluminescent technology.
Interpretation Measurement of serum Phenobarbital concentrations may be useful:

* to evaluate compliance or toxicity

* to determine if the drug has been given an adequate trial before switching to or adding another anticonvulsant

* when a potentially interacting drug (phenytoin, valproic acid) is given.

Because of the long elimination half-life and time to steady state (1-2 weeks in children, 2.5 to 3 weeks in adults), phenobarbital levels may be obtained before steady state is achieved, thus underestimating true steady state concentrations.

Toxic symptoms such as sedation and ataxia may be seen above 35 mg/L (> 151 mcmol/L). Coma may be seen at levels above 65 mg/L (> 280 mcmol/L) in non-tolerant persons, although some tolerant persons may have concentrations as high as 90 mg/L with few symptoms.
Container type gold top gel tube
Amount to Collect 2 mL
Collection Instructions Plasma samples cannot be assayed.
Sample type Blood
Normal range 15 - 40 mg/L (65-172 mcmol/L) serum for anticonvulsant therapy.

All values greater than 50 mg/L (> 216 mcmol/L) are considered Critical Values and are called to the physician or patient care unit immediately.
Stability 24 hours at 4°C; 1 month at -20°C.
Interferences 1. Primidone is metabolized to phenobarbital in vivo.

2. 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).
References 1. Penry, J.K. and Newmark, M.E.: Ann Int Med 90:207, 1979.

2. Bayer 120518 Rev. C, 2002-07. Siemens Medical Solutions Diagnostics, Malvern, PA.

3. Drug Monitoring Pocket Guide II. AACC Press 1994. American Association for Clinical Chemistry, Washington, DC.
CPT coding 80184
Last Updated 10/22/2012 7:48:32 PM
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