A patient with chronic renal failure did not respond to “therapeutic” serum phenytoin concentrations as measured by enzyme-multiplied immunoassay (EMIT®). Her seizure disorder was not controlled despite phenytoin concentrations of 25–35 μg/ml. When her serum samples were reassayed by high pressure liquid chromatography and gas-liquid chromatography, the phenytoin serum concentrations were found to be approximately J/a of the previously reported values, that is, 16.0 μg/ml vs. 33 μg/ml. The precise reason for this discrepancy is currently unknown. Our findings suggest that the EMIT® assay may yield falsely elevated serum phenytoin concentrations in uremic subjects.
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