Elevated serum phenytoin concentrations in a uremic patient when measured by enzyme-multiplied immunoassay.

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 micrograms/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 1/2 of the previously reported values, that is, 16.0 micrograms/ml vs. 33 micrograms/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.