Simultaneous measurement of phenobarbital, diphenylhydantoin, and primidone in blood by high-pressure liquid chromatography.

We describe a sensitive, precise high-pressure liquid chromatographic method in which 5-(p-methylphenyl)-5-phenylhydantoin is used as the internal standard for simultaneous determination of diphenylhydantoin, phenobarbital, and primidone in whole blood and plasma. These anticonvulsant drugs are well separated from each other and from normal blood constituents in less than 10 min. The lower limit of detection for each drug is 100 ng for primidone, 200 ng for dilantin, and 300 ng for phenobarbital. The eluted drugs were detected by their absorption at 254 nm, and evaluated from their peak heights as compared to internal standard. The method was successfully adapted for pediatric samples (100 to 500 mul of whole blood or plasma). Fifty specimens were analyzed for phenobarbital and diphenylhydantoin and 25 specimens for primidone by a standard gas-chromatographic method and by our liquid-chromatographic method; the resulting correlation coefficient was greater than 0.98.