Determination of diphenylhydantoin in human serum by spin immunoassay.

A spinn immunoassay for diphenylhydantoin is reported, which appears to give an accurate and precise estimate of serum diphenylhydantoin concentrations, as judged by the disappearance of [14C]diphenylhydantoin from the serum of a rabbit. The assay also appears to be a reliable technique for routine diphenylhydantoin determinations, as judged from our experience with 28 patients. Serum diphenylhydantoin concentrations in the range of 1.0-50.0 mg/liter are easily determined on a 50-mul sample. Except for primidone, no significant cross reactivity was observed with eight drugs that are commonly used in conjunction with diphenylhydantoin therapy. This fast, simple, and precise method therefore appears to be readily applicable to routine determination of diphenylhydantoin.