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Test Code PHENO Phenobarbital Level, LAB30


Turbidimetric Inhibition Immunoassay

Performing Laboratory

St. Luke's Hospital Cedar Rapids

Specimen Requirements


Preferred: Green-top (heparin) tube

Acceptable: Gold-top serum gel tube or plain red-top tube

Specimen Volume: 0.2 mL of plasma or serum

Stability: Separated specimens should be analyzed immediately or kept at room temperature

               for 2 days or 2 – 8°C and analyzed within 30 days. For longer storage, samples may

               be frozen at -20 °C for up to 3 months.

Collection Instructions: Peak level should be drawn 1 to 3 hours post dose.

                                         Trough level should be drawn immediately prior to next dose.

Note: 1. Phenobarbital time to steady state is after 10 to 25 days of chronic oral dosing for adults and after

              8 to 20 days of chronic oral dosing for children.

          2. Levels may be ordered at any time toxicity is suspected.

          3. Label specimen appropriately (plasma).



Reference Values

Therapeutic range: 15.0-40.0 mcg/mL

Critical value: >40.0 mcg/mL

Day(s) Test Set Up

Monday through Sunday

Test Classification and CPT Coding


Useful For

Phenobarbital is an anticonvulsant and sedative-hypnotic drug used in the treatment of epilepsy, in particular for controlling focal motor or sensory seizures and grand mal seizures. 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.