Phenobarbital Level

Abbrev Code:PHENO   
Order Code:LAB30Order Name:Phenobarbital Level
Synonyms:Also see Free Phenobarbital (FPHENO, LAB3482)
Methodology:Kinetic interaction of microparticles in a solution (KIMS)
CPT Codes: 80184 x1, 80184 x1
Turnaround Time:Performed and reported 24 h/day.

Collection Instructions

Optimal Volume:2.5 mL
Minimum\Peds Volume:0.8 mL
Container:Red (no gel)
Alternate Containers: Purple (EDTA)
Collection Instructions:Trough level is preferred; collect immediately before dose. It is advisable to monitor at the same time in the dosing schedule.
Causes for Rejection:Collection in a gel tube.

Processing and Shipping

Specimen Processing:Centrifuge and aliquot 1.25 mL, 0.3 mL minimum. Store in room temperature up to 48 hours. Alternatively, refrigerate.
Shipping Instructions:Ship at room temperature to arrive within 48 hours. Alternatively, ship at refrigerated temperature.
Stability:7 days at room temperature; 7 days refrigerated; 1 year frozen.
Test Performed at or Referral Lab Chemistry  (Roche-Grand Itasca, Roche-Range, Roche-UMMC East)


Critical Range:Greater than 50 ug/mL
Therapeutic Range:10-30 ug/mL

The therapeutic range of phenobarbital is correlated with seizure control as well as the absence of toxic effects, and is generally accepted to be between 10 and 30 ug/mL. Variation in metabolism and absorption of the drug may cause levels to rise above 40 ug/mL or fall below 15 ug/mL.  The most frequent dose related side effect is sedation, to which a tolerance usually develops. Phenobarbital serum levels above 40 ug/mL are often associated with nystagmus, ataxia and dysarthria. At high doses, phenobarbital can cause an increase in seizure frequency.
Use:Monitor therapeutic drug level of phenobarbital; use with primidone levels to monitor therapeutic drug level of primidone; evaluate phenobarbital toxicity. Half-life is 50-120 h (adult); and 40-70 h (child).

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