Phenobarbital, plasma or serum

Phenobarbital is the most effective barbiturate in long-term treatment of seizure disorders. Strong correlation has been observed between serum levels and both therapeutic and toxic effects. Phenobarbital is excreted, partly metabolized and partly unchanged, very slowly through the urine. Both metabolism and clearance are highly variable among individuals; therefore, close monitoring of serum levels is essential for every patient undergoing phenobarbital therapy. Clinical observations indicate that toxicity of phenobarbital is increased in patients with renal disease. Phenobarbital toxicity primarily affects the central nervous system. Some patients on long term therapy may develop macrocytosis, megaloblastic anemia and osteomalacia.