Phenobarbital (PHB)

Clinical Biochemistry

Phenobarbital is now a minor anticonvulsant drug, being rarely used in Europe. The pro-drug primidone is never now used.
Phenobarbital is a third line drug for tonic or partial seizures and is also used for the prophylaxis of febrile convulsions in children at particularly high risk.
Routine monitoring is of little value, but may be useful to check compliance, to minimise the effect of drug interactions, or to confirm a clinical diagnosis of toxicity.
Monitoring is of value, however, in children receiving the drug for treatment of febrile convulsions to ensure adequate levels attained.
Phenobarbital has a long half-life so precise timing of sampling is not required, pre-dose (trough samples) are mostly used.

For adults, blood taken into a 5mL gold top gel tube (rust top for the Acute Unit)

For children, blood taken into 3.5mL rust top tube

For infants/neonates, blood taken into a 1mL red top tube

Send at ambient temperature to the laboratory. If unavoidable, samples can be stored refrigerated overnight.

Relevant clinical details including reason for the request, date and time of last dose, date and time of sample.

Samples are referred to an external laboratory for analysis with results expected back within 7 working days.

Ranges and interpretation reported as provided by referral laboratory.

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Page last updated: 29/09/2025 | Page last reviewed: 29/09/2025