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Phenobarbital (PHEN)

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EPIC Test Name

PHENOBARBITAL LEVEL

EPIC Code

LAB30

Specimen Requirements

plasma
Minimum Volume:0.5 mL
Collection:Collect using standard laboratory procedures
Transport:Room Temperature ASAP
Stability:Room Temperature: 7 days capped at 15-25 degrees C Refrigerated: 7 days capped at 2-8 degrees C
Frozen: 1 year capped at -20 degrees C
Container:LIT-GRN
Rejection Causes:Insufficent Sample Volume

Methods

Kinetic Interaction of Microparticles(KIMS)

Turnaround Time

SpecimenTurnaround TimeFrequency
plasmaStat: 90 minutes Routine: 4 hours24/7

Reference Ranges

Kinetic Interaction of Microparticles(KIMS)
UnitCritical ValuesTherapeutic Levels
µg/mL>60 ug/mL15.0-40.0 ug/mL

Clinical Indications

Phenobarbital is used in the treatment of all seizures, e.g., generalized tonic-clonic, partial, focal motor, temporal lobe, and febrile seizures, except absence seizure. The mechanism of its action is known for reducing synaptic transmission, resulting in decreased excitability of the entire nerve cell and a consequent sedating effect. It potentiates synaptic inhibition through action on the GABA-A receptor by increasing the duration of chloride flow into the synapse. The overall outcomes are an increase in seizure threshold and inhibition of the spread of discharges from the epileptic foci.
Absorption of oral phenobarbital is slow but complete with peak plasma concentrations reached from 4 to 10 hours after the dose. Its bioavailability of oral administration is >90%. About 40% to 60% of phenobarbital are bound to plasma proteins. The elimination half-life varies from 70 to 100 hours, depending on age (children average 70 hours, and geriatric patients 100 hours). Due to the long elimination half-life, its blood concentration does not change rapidly. Therefore, a trough value is representative of the overall effect. Results from specimens collected 2 to 4 hours after dosing can be construed to be the peak concentration, and may causing misleading in dosing adjustment. Its therapeutic concentration is between 15 and 40 µg/mL (66 to 177 µmol/L). The main side effect in adults at concentrations > 40 µg/mL (177 µmol/L) is sedation, tolerance to this effect may develop with chronic therapy.
Phenobarbital is metabolized in the liver to p-hydroxy phenobarbital, which is mainly excreted in urine as the glucuronide or sulfate ester. When renal and hepatic functions are impaired, patients can experience decreased clearance of the drug. In chronic administration of either valproate or salicylate, phenobarbital concentrations may increase by 10% - 20% and a dose adjustment may be necessary to avoid intoxication. Phenobarbital induces mixed-function oxidative enzymes, causing increased metabolism of other xenobiotics after about a 1- 2 week therapy.

Performed

Lab
Chemistry - Downtown

Interpretative Information

Dose should be adjusted to achieve steady-state within the therapeutic range, mostly between 10.0 and 40.0 mcg/mL.

CPT

80184

LOINC

3948-7

References

1. Faero O, Kastrup KW, Lykkegaard Nielsen E, et al.: Successful prophylaxis of febrile convulsions with phenobarbital. Epilepsia 1972;13:279-85. PMID: 4615893
2. Milone MC, Shaw LM: Therapeutic drugs and their management. In: Rifai N, Horvath AR, Wittwer CT, eds: Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018:800-831.

Contact Information

Chemistry - Downtown: (315)464-4460
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