Phenobarbital

Test Code

PHENB

Alias/See Also

PHNO, Pheno

Preferred Specimen

1.0 mL plasma (0.5 mL minimum)

Instructions

Collect trough just before next dose.

Transport Container

PST (light green top). Red top and SST also acceptable

Specimen Stability

Room temperature: 2 days, Refrigerated: 1 month, Frozen: 3 months

Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)

Grossly hemolyzed specimens are unacceptable.

Limitations

"Although amobarbital does not interfere in the therapeutic range, a toxic level of amobarbital will increase the response of phenobarbital.
Mephobarbital is rapidly metabolized in vivo to phenobarbital, its active metabolite."

Reference Range

Therapeutic: 15.0 - 40.0 ug/mL

Clinical Significance

Phenobarbital test results are used in monitoring levels of phenobarbital to ensure appropriate therapy and in the diagnosis and treatment of phenobarbital overdose. Phenobarbital is particularly useful in grand mal and focal seizures and in seizures due to withdrawal of alcohol or barbiturates. Petit mal and psychomotor seizures do not respond to this drug and may be worsened. Because of considerable inter-individual variation and the limited capacity for the liver to metabolize phenobarbital, blood concentrations should be monitored to obtain maximal anti-seizure effect. Once metabolism is saturated, small dosage changes may result in disproportionate changes in blood concentration and can cause wide variations in dosage requirements among patients.

The CPT Codes provided in this document are based on AMA guidelines and are for informational purposes only. CPT coding is the sole responsibility of the billing party. Please direct any questions regarding coding to the payor being billed. Any Profile/panel component may be ordered separately. Reflex tests are performed at an additional charge.