The preferred specimen for phenobarbital is 1-2mL of clear serum drawn in a plain red top tube, collected 4 hours after administration of phenobarbital (peak measurement). Transfer serum into a clean, plain red top transport tube as soon as possible. Phenobarbital concentrations can be falsely decreased with the use of serum separator tubes (gel tubes) and inaccurate with markedly hemolyzed or icteric samples.
Plastic or glass tube
Label all samples. Ship on cold packs. Ship overnight.