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May 1976

Interlaboratory Variability in Determination of Plasma Antiepileptic Drug Concentrations

Author Affiliations

From the Department of Neurology, College of Physicians and Surgeons, Columbia University, New York (Dr Pippenger), the Neurological Disorders Program, National Institute of Neurological and Communicative Disorders and Stroke, Bethesda, Md (Dr Penry and Mr White), the Department of Neurology, Health Science Center, University of Texas Southwestern Medical School, Dallas (Dr Daly), and the Epilepsy Foundation of America, Washington, DC, (Dr Buddington).

Arch Neurol. 1976;33(5):351-355. doi:10.1001/archneur.1976.00500050037007

• The usefulness of plasma antiepileptic drug concentrations in treatment of epilepsy has been established, and many laboratories provide this service. A "blind" survey utilizing pooled patient plasma samples was conducted among 197 laboratories in the United States and Canada to establish the interlaboratory reproducibility. Three "patient specimens" containing different amounts of phenobarbital, phenytoin (diphenylhydantoin), primidone, and ethosuximide were employed; 112 laboratories reported results within five weeks. The average cost for analyzing four drugs in a single sample was $43.27. Half of the laboratories reported results outside ± 1 standard deviation of the mean of five reference laboratories. Wide interlaboratory variability must be considered by the practicing physician. Until certified antiepileptic drug standards in a biologic matrix are available from the National Bureau of Standards, a volunteer quality control program among laboratories is needed.