Preparation for Phenobarbital Assay
No special patient preparation is required for phenobarbital serum level testing, and timing of blood collection is not critical in patients on chronic therapy.
Timing of Sample Collection
Blood samples can be collected at any time during the dosing interval in patients on established phenobarbital therapy. Research demonstrates that in patients receiving phenobarbital for ≥3 weeks, there are no therapeutically relevant changes in serum concentrations throughout the daily dosing interval 1. In a study of epileptic dogs on chronic phenobarbital therapy, 91% had trough, 3-hour, and 6-hour serum concentrations within the same therapeutic category, with no significant differences detected among mean concentrations at these time points 1.
Sample Type and Collection
- Whole blood (fingerstick) or venous plasma samples are both acceptable for phenobarbital measurement 2
- Whole-blood results can be reported in plasma equivalent values using modern immunochromatographic assays 2
- The assay is appropriate for use in offices, outpatient settings, and emergency rooms 2
Therapeutic Range and Interpretation
- The therapeutic anticonvulsant serum level is 10 to 25 µg/mL 3
- In neonates, serum concentrations up to 40 µg/mL may be used, with refractory seizures potentially requiring levels up to 100 µg/mL 4
- For toxicity assessment, serum concentrations >50 mg/L may induce coma and concentrations >80 mg/L may be fatal 5
Special Considerations
No fasting or dietary restrictions are necessary prior to phenobarbital level testing. The drug undergoes hepatic metabolism with approximately 25% excreted unchanged in urine 4, 6. Phenobarbital is not detected in cerebrospinal fluid or feces in clinically significant amounts 6.
Clinical Context for Testing
When evaluating phenobarbital levels for toxicity or overdose management, simultaneous urine drug screen and blood ethanol concentration should be obtained to confirm presence of co-ingested drugs that could influence clinical assessment 5.